Mario Marcelo Santoro left Australia for Brazil the weekend Ms Haddad's body was found in a Sydney river in April and police on Wednesday said the warrant had been secured.

But the warrant, which means he would be charged with murder if he was in Australia, doesn't guarantee Brazilian authorities will hand him over to face charges.

"Our agreement with Brazil says there is no obligation to extradite a national," international lawyer Rowan Nicholson told AAP on Wednesday.

But it allows the Australian government to negotiate with Brazil to bring Santoro to justice over the murder of one its own citizens.

Dr Nicholson said this is generally the case in civil law countries where the legal system operates differently to Australia, the UK and the US.

"If Brazil refuses to extradite a national on that ground, then the treaty contemplates that it will submit the case to its own authorities to prosecute that person in Brazil," the University of Sydney associate lecturer said.

The federal Attorney-General's department said it does not disclose whether it has made or intends to make an extradition request until the person is arrested or brought before the court in a foreign country.

Ms Haddad, 38, had told her former live-in lover Santoro to move out of her Ryde apartment weeks before she disappeared, detectives revealed in May.

Her fully-clothed body was found in the Lane Cove River on Sunday, April 29, about the same time Santoro flew to Brazil.

Premier Gladys Berejiklian said the NSW government would work co-operatively with police.

"We want to make sure justice is served," she said.

A relative of Santoro last month claimed he planned to provide documents proving he returned to Rio de Janeiro because his father was ill in hospital.

Police have previously said there were no clear signs of a disturbance at Ms Haddad's home.

The Brazilian-born executive arrived in Australia more than a decade ago and moved to Sydney in 2016, where she worked in mining and logistics.

Ms Haddad had been at a barbecue the Friday night before she vanished and made her last call to a friend the following morning.

When she failed to keep appointments later in the day her friends raised the alarm before kayakers discovered Ms Haddad's body.

1800 RESPECT (1800 737 732)

Lifeline 13 11 14

Australian Associated Press