As Liu pledged her fidelity to David Shultz, and Xu spoke of his devotion to Grace Clague, the two couples committed themselves to a lie that would stay hidden for years. At the time of the weddings, believed to have been in 1989, all parties were relative nobodies. But as the decades passed, the stakes for one participant, Liu, got far higher. As a multimillionaire with impressive links to the Labor Party, especially to federal MP Joel Fitzgibbon, and ties to top Chinese officials in Beijing, Liu had become a somebody.

In March 2009, Liu's long-standing friendship with Fitzgibbon and financial support for him became public. He was defence minister at the time and had been living in a Canberra property owned by Liu's family.

After initially denying having received any substantial gifts from Liu, Fitzgibbon was forced to admit he had received two all-expenses-paid trips to China while an opposition MP and had failed to declare them to Parliament. He quit cabinet months later after it was revealed his brother had used his ministerial office to lobby for defence health contracts.

Amid the publicity in 2009, Liu highlighted her status as an Australian citizen, saying, ''I am an Australian citizen and I participate in all activities, not just political … I am a very good Australian businesswoman''.

Fitzgibbon and former NSW premier and now Foreign Minister, Bob Carr, also came to her defence, with Carr describing his friend as a nice, pleasant and slightly shy woman, according to one media report.

Carr took umbrage at media reports raising national security concerns regarding Liu's connections in Beijing, suggesting they were based on Sinophobia. ''I think Helen Liu is entitled to object to being branded a 'mystery woman' - she's no more a mystery than any other woman but for her anonymity,'' Carr said.

Maybe Carr was not aware of the biggest mystery of all surrounding his friend: how Liu attained permanent residency and citizenship.

Today, that mystery comes to an end, with one of the participants in the sham weddings, Clague, revealing that she and Shultz - her boyfriend at the time and later her husband - entered into the marriages because of a mistaken belief that Liu and Xu's lives would be in danger if they returned home.

Clague was aged 18 at the time of her marriage. Shultz was 20. Clague says they were ''young, naive and stupid'' when they agreed to take part in a sham with potentially serious criminal consequences. She says she and Shultz became friends with Xu and Liu after being introduced to the couple by an Australian friend, Philip Everson, who worked with a Sydney migration agent at the time. Xu and Liu were in Australia on student visas.

''I [It's] corny, I know, but that was me. I saw their love, I wanted to be part of it and share it … the final decision was made when I was told their lives were at stake,'' says Clague, the daughter of prominent Aboriginal activist Joyce Clague and NSW state Labor election candidate Colin Clague.

The marriages were never consummated and the wedded couples never lived together. Clague says that after the wedding ceremonies, she and Shultz only occasionally saw Xu and, even less frequently, Liu.

''I did [catch up] with Harry a bit. I don't remember Helen being around so much. I thought that was because she wasn't at home or whatever,'' she says.

Clague and Shultz had two of their three children while still technically married to their respective Chinese spouses. After those marriages ended, Clague and Shultz wed each other. They have since separated.

For their part, Liu and Xu got busy amassing a Sydney property empire worth $40 million, using money and loans sourced from China and the Bank of China.

They were so busy, in fact, that they forgot to divorce the young Australians until about 1995, about three years after being granted permanent residency. The realisation that the Australians were legally entitled to half their fortunes may have prompted the Chinese pair to finally end the marriages.

Liu and Xu also built a network of useful contacts in the NSW Labor Party, chiefly federal government MP Eric Fitzgibbon and his son and successor as the member for Hunter, Joel Fitzgibbon.

As early as June 1993, one of Liu's companies paid for Eric and Joel Fitzgibbon to fly first class to mark the start of work on a tourism development in Liu's home province of Shandong. During their seven-day stay, the Fitzgibbons were guests of the provincial governor and top Communist Party official Zhao Zhihao and Ms Liu's company Diamond Hill International.

It was a remarkable achievement for someone who four years earlier, according to Clague, claimed to be at risk of persecution.

The first inkling the Department of Immigration and Citizenship got that there were serious doubts over the validity of Liu's marriage to Shultz came in the form of a letter sent by opposition immigration spokesman Scott Morrison about 12 months ago. Morrison requested that departmental secretary Martin Bowles investigate the marriages, citing potential ''migration fraud''.

Morrison's letter came after Fairfax Media provided his office with information that cast doubt over the legitimacy of Liu's marriage to the Australian man.

Migration fraud and making false declarations to the Commonwealth are both criminal offences punishable by significant jail terms or hefty fines. In seeking permanent residency, Liu and Xu had to make sworn statements to the Commonwealth declaring a ''genuine and long-term commitment'' to an Australian national.

But instead of properly investigating Morrison's concerns, Immigration officials aborted their inquiry, claiming there was ''insufficient evidence'' to proceed.

Yet it appears that before deciding to shelve the probe, officials made no effort to locate the key players, including Liu, who declined to answer questions about her marriage for this story.

Morrison says: ''I referred a serious matter to the Immigration Department. I expected them to investigate it, not bury it.''

Although the department claims to have a ''voluminous'' amount of documents relating to its response to Morrison's request for an investigation, it has refused to release any under freedom of information laws for reasons of personal privacy. A small number of documents released in response to a separate FOI request show the department regarded the Liu case as warranting special security arrangements in regard to which officials could access its files.

The documents that have been released reveal Immigration's view that the matter was ''highly sensitive'' because of Liu's connections to Fitzgibbon, who at the time of the request was chief government whip, and to his father.

They also show that the office of former immigration minister Chris Bowen was consulted by the department on its decision not to release information concerning Liu and Xu. Bowen is a friend of Fitzgibbon, and both are members of the NSW Labor Right faction.

Had Immigration officials bothered to properly investigate they might have discovered that one of the key participants in the sham weddings, Harry Xu, has long maintained that, despite his marriage to Clague, he and Liu were in a consistent de facto relationship from 1988 until 1994.

When Xu and Liu's personal relationship broke down in 1994, their business partnership also soured. The former couple did battle in the Federal Court and NSW Supreme Court in 1997 and 1998 over who had the rights to their companies' multimillion-dollar properties.

Xu's claims were supported by a signed statement from a Sydney landlord, who said the Chinese pair lived together from March 1989 into 1990. In reply, Liu swore an affidavit in the Federal Court denying they had had a long-term de facto relationship and only admitting to living with him for a brief period in 1993. She did not refer in her affidavit to her marriage to Shultz as a means of rebuttal.

Liu's affidavit did, however, provide a glimpse of her family's connections in China. Her father was described as a ''ranking official'' in the Chinese government and reference was made to other relatives in high-level government positions.

Fairfax Media has confirmed two of the migration agents involved in the marriage of Xu and Liu to the Australians or in the submission of their residency applications have been de-registered by the Immigration Department.

Department records of interviews, obtained under freedom of information laws, show John Lingham, the migration agent whose office submitted Liu and Xu's residency application paperwork, regularly advertised his services in Sydney's Chinese-language newspapers in the late 1980s and 1990s.

Lingham says he believes the paperwork for Liu and Xu's residency applications would have been submitted under the letterhead of his firm at the time, Lingham & Associates. However, he says he believes his then employee, Philip Everson - who was friends with the young Australians, Shultz and Clague - prepared the paperwork.

When contacted by Fairfax Media, Everson denied knowing anything about the marriages.

Court records of other migration fraud prosecutions show young Australians taking part in sham marriages between 1989 and 1991 could earn thousands of dollars. But Clague says neither she nor Shultz received any money to marry the Chinese pair. Shultz declined to speak to Fairfax Media.

Clague insists her intentions at the time of the marriages were good.''They seemed honest, caring people and I believed their lives were at stake,'' she says.