JULIAN Assange may be safe from extradition to the United States even if he returns to Sweden, suggests one of the Scandinavian country's top judges.

In a rare public lecture delivered in Adelaide, Justice Stefan Lindskog defended the leaking of classified information, saying the case against the WikiLeaks founder was "a mess", and raised many questions over the legality of the US ever being able to extradite Assange via Sweden.

"It should never be a crime to make known crime of a state," Justice Lindskog, the chairman of the Supreme Court of Sweden, told a crowd at the University of Adelaide on Wednesday night.

Swedish prosecutors are pursuing Assange for questioning over allegations of sex offences against two women.

Assange is living in the Ecuadorian embassy in London where he has been granted political asylum after failing to resist moves to extradite him to Sweden.

He fears if he goes to Sweden he will be at risk of extradition to the US to face conspiracy or other charges arising from WikiLeaks obtaining thousands of secret US military and diplomatic reports.

But Justice Lindskog, who went into an extraordinary amount of detail on Assange's Swedish sexual assault case, said the extradition treaty may see the courts rule against sending Assange to the US.

"Extradition shall not be granted when alleged crimes (are) military or political in nature," he said.

He also said extradition could only be granted if the Swedish courts would hear a similar case to the charges being brought by the other country - and it was debatable if Mr Assange would have committed a crime under Swedish law.

"What is classified under US law is probably not classified under Swedish law, and enemies to the US may not be enemies to Sweden," he said.

Thirdly, Swedish law protected sources who leak to the press, he said, and that protection meant a prosecution would likely not go ahead in Sweden and therefore may not be grounds for extradition.

He added that extensive media coverage of the case has simply led to distrust in the legal system.

"I think it is a mess," he said.

Justice Lindskog also backed a suspected source to WikiLeaks, US soldier Bradley Manning.

He said the release of classified information was for the benefit of mankind - especially secret combat video in Iraq that showed the American crew mowing down a group of civilians and a Reuters photographer.

He said he hoped Mr Manning would have a fair trial.

Prior to the speech, Assange had condemned Justice Lindskog's decision to speak in Australia, calling it "absolutely outrageous".

But George Williams, a University of New South Wales law professor who took part in a panel discussion after the speech, said it was important to remember the judge wasn't sitting on the case.

"This is a little unusual, but different countries have different standards when it comes to public comment on cases," he told AAP earlier in the week.

In February, Foreign Minister Bob Carr had stressed many of the same points as Justice Lindskog, saying it was "sheer fantasy" to think the US could ever extradite Mr Assange from Sweden.

Originally published as Top Swedish judge backs WikiLeaks