Thi Tran, 45, a mother of two teenage children, and Manh Dao, 36, the father of a young child, will spend at least 13 years in prison after a police patrol happened on them by chance in a park in Maribyrnong, a Melbourne suburb, two years ago.

The pair were thought to be acting suspiciously when they walked away from a black BMW as soon as they saw a police patrol drive past Jensen Reserve about 4.20am on July 13, 2011, at the end of a separate police investigation into drug trafficking.

A police search of the car uncovered 25.2 kilograms of high-purity heroin and 6.6 kilograms of ice in suitcases, plus $160,000 in cash. Police estimated the drugs could have fetched up to $17 million wholesale or $47 million if sold on the street.

Dao and Tran initially denied any link to the car until it was found to be registered in the name of the man's mother-in-law and the woman's handbag and several forms of identification were found inside.

The pair pleaded guilty in the County Court to one charge each of trafficking a large quantity of drugs of dependence after initially fighting the charges in a trial, before changing their pleas.

Judge Michael Tinney said Dao and Tran were guilty of a "most serious example of a most serious offence", and said he had no choice but to impose long sentences because of the quantity of drugs, the lateness of their guilty pleas and their lack of remorse.

"These drugs were worth an enormous amount of money," he said. "In any form, however sold, they were worth many, many millions of dollars and would have ultimately provided several hundreds of thousands of deals or hits, with untold misery and impact on users and the community."

The judge described as unsatisfactory the pair's refusal to explain how and from whom they obtained the drugs, their destination, how much they were to be paid and even how they knew each other.

He said he could not tell if the pair were trying to protect themselves or others. "I am being asked to speculate, I am being asked to guess about your roles and I won't," he said.

Judge Tinney said there were no mitigating circumstances worthy of consideration for the pair, who were educated to the equivalent of year 12 in their native Vietnam before moving to Australia.

But he said their isolation and lack of English would make their time in jail more onerous.

Judge Tinney said he had considered bringing a contempt charge against one of Tran's brothers-in-law, who had tried recording an earlier court hearing on his iPhone, but had decided against further action.