Former NSW premier and foreign affairs minister Bob Carr says Julian Assange's extradition from London would create a precedent allowing other countries to do the same to Australians overseas.
Mr Carr is one of almost 400,000 people who have signed an Amnesty International petition calling on the US to drop charges against the Wikileaks founder.
It was on Tuesday handed to the US consulate in Sydney, coinciding with the incarcerated Australian's extradition hearing in London.
Assange is fighting US prosecutors' attempt to send him to America to stand trial on spying charges.
"The arguments for Australia doing something here are pretty clear cut," Mr Carr told reporters.
"He faces the prospect of a living death inside an American prison ... in very cruel conditions because he let the world know about an American war crime in Iraq."
Mr Carr said if he were foreign minister he would be "unabashed" in asking US Secretary of State Mick Pompeo to drop the extradition proceedings.
"He didn't commit these crimes in America. Everyone concedes he not a spy, he hasn't been committing espionage," Mr Carr said.
"If the Americans can get away with this - digging up an Australian in London and putting him on trial for breaching their laws. Why can't another government do the same thing?
"For example, an Australian who's campaigning for human rights in Myanmar.
"That Australian in theory could be sought by the government of Myanmar to be taken back to Myanmar from London and to be put on trial there for breach of their national security statues.
"This is a very bad precedent."
A British judge overnight rejected a request by his lawyers to delay Assange's extradition hearing until next year, to give them time to respond to US allegations that he conspired with hackers to obtain classified information.
US prosecutors have indicted the 49-year-old on 18 espionage and computer misuse charges over WikiLeaks' publication of secret US military documents a decade ago.
The charges carry a maximum sentence of 175 years in prison.
Assange's lawyers say the prosecution is a politically motivated abuse of power that will stifle press freedom and put journalists around the world at risk.
The case is due to run until early October.