A Federal Court judge "exceeded his power" and was "flawed" in his decision to put a stop to the extradition of elderly Willetton man Charles Zentai, who Hungary wants to prosecute over war crime allegations, a Commonwealth lawyer argued in an appeal hearing today.

The Federal Government's two-day appeal against the ruling of Federal Court Justice Neil McKerracher that Mr Zentai, 89, was not eligible for extradition started in the Federal Court today.

Justice McKerracher's ruling overruled a previous Federal Court decision to grant the extradition.

Mr Zentai denies allegations he was involved in the murder of a Jewish teenager when he was a soldier in the German-aligned Hungary army in 1944.

As reported in The West Australian, Mr Zentai was not in court today because he is recovering at home and seeing his GP after suffering a minor stroke on Friday, which saw him lose his speech temporarily and be hospitalised.

Commonwealth lawyer Stephen Lloyd, representing Home Affairs Minister Brendan O'Connor, argued Mr Zentai fitted the legal definition of an accused person and it was not necessary for a prosecution to have commenced for there to be extradition, only for a warrant of arrest to be issued.

Mr Lloyd said Mr Zentai was under "grave suspicion" of being involved in the alleged murder and that it was the intention of Hungarian authorities to put Mr Zentai to trial unless he could dissuade them.

He said under Hungarian criminal procedure, before the matter proceeded to a military panel Mr Zentai would have the chance to put his case forward which could rule out the need for a prosecution.

Mr Lloyd said claims the extradition was sought only so Mr Zentai could assist with preliminary investigations was wrong.

Outside court today, Mr Zentai's son Ernie Steiner said Hungarian authorities have never categorically said they wanted to prosecute his father, only to "question him in the interest of the case."

He said his father, who he claimed was not even in Budapest on the day of the murder, was willing to be questioned by Hungarian authorities in Australia.

"He's got nothing to hide, he's passed a lie detector test, he's given evidence to the minister, they won't look at it, they won't talk about innocence," he said.

"It's inhumane, when all of this could be handled here in Australia... it will kill him, he just can't survive, the stress of even something like today is enormous.

"Imagine facing imprisonment in Hungary for an indefinite term while the Hungarian Government decides what they were going to do with you ... their whole process as you've heard today is not clear."

Mr Steiner said his father was improving after Friday's mini-stroke, which was the latest and most serious in a series of minor strokes, but he was still not remembering a lot of words.

In March, the Federal Government confirmed its legal bill for this matter, not including the current appeal, was about $455,000.

Mr Steiner said his father and the family had spent well in excess of $200,000 fighting the extradition, until a pro bono arrangement was made with their current legal team, led by prominent barrister and soon-to-be WA Governor Malcolm McCusker.

Mr McCusker sought to introduce new evidence explaining the reasons behind his client's surname being changed from Steiner to Zentai.

He said the surname was changed when Mr Zentai was 13 and assertions it was changed to escape detection and evade authorities were "demonstrably false."

The Commonwealth told the court today it did not take any issue with the reason behind the name change.

Mr Steiner said his father's older sister changed the family name because people were encouraged to revert back to their Hungarian surnames.

The appeal before interstate Federal Court judges Anthony North, Anthony Besanko and Christopher Jessup continues.

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