David Peter Kisun, 77, was found guilty of four counts of indecent assault in the ACT Supreme Court in November 2018.
He was handed a 28-month suspended sentence for his conduct against two students at Marist College in Canberra in the 1980s.
Kisun is now fighting extradition to New Zealand over allegations he committed sexual offences against two other students at a Marist Brothers school in Wellington from 1969 to 1971.
In November 2023, magistrate Christopher McRobert signed a warrant surrendering Kisun to New Zealand and sending the convicted child sex offender into custody awaiting his departure from Australia.
The 77-year-old failed in getting bail in December and brought his challenge to those orders to a head at a Federal Court hearing on Thursday.
Kisun's solicitor Greg Walsh argued his client was mentally unfit to stand trial in New Zealand.
"The evidence is overwhelming that this man has a very significant cognitive decline consistent with suffering from dementia," he told Justice Nye Perram.
Kisun had not been diagnosed with dementia despite "extraordinary efforts" to do so, Mr Walsh said.
Several attempts to get an MRI scan or an appointment with a neurologist had been thwarted because he was in custody, the court heard.
The former teacher's mental state plus a delay of over 50 years in bringing the charges also meant he was no longer able to properly defend himself, his lawyer said.
Mr Walsh submitted that the New Zealand criminal process operated at a lower standard than that of NSW.
In particular, he said those mentally unfit to stand trial in New Zealand would go through a special hearing, where the standard of proof was the balance of probabilities rather than the higher bar of beyond reasonable doubt.
"The quality of trial according to Australian standards is the test," he said.
Mr Walsh argued Kisun could not be extradited because he potentially faced "representative charges" in New Zealand.
When an alleged victim says they were sexually assaulted many times over a period of time but is unable to say exactly when that happened, prosecutors can elect to pursue one charge representing the wider conduct in court.
Mr Walsh said these types of charges were permitted in New Zealand but not in NSW and a court should not agree to the extradition as a result.
Barrister Thomas Muir, representing the New Zealand government, supported the extradition, saying there would be no injustice to Kisun.
"He will receive a fair trial," he said.
While there was evidence that Kisun had a "mild cognitive impairment", there was nothing to prove he had been diagnosed with dementia, Mr Muir said.
The Australian Federal Police officer who spoke to Kisun on his arrest in September 2023 said the ex-teacher appeared to understand what was going on, Justice Perram was told.
The hearing continues.
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