John Shipton is in London where the High Court on Wednesday will begin a two-day hearing to consider an appeal lodged by American authorities.

"I imagine that they'll support the decision of non extradition," he told AAP of the High Court hearing.

Mr Shipton said the bench will comprise the most powerful judges across England and Wales, adding that their ruling will be "unappealable".

The US wants Assange on their soil to face charges of violating the Espionage Act and publishing secret American documents through WikiLeaks.

In May 2019, the government charged him with 17 counts of trying to hack a Pentagon computer and offences under the Espionage Act, that carry penalties of 175 years in prison.

Mr Shipton wants his son returned to Australia with the support of Canberra.

"Bring him to Australia and say 'look, you're free here, there will be no more, we will resist the persecution'," he said.

Mr Shipton said the US is relying in part on falsified claims in its case against his son and Assange's lawyer Jennifer Robinson added it was time the prosecution ended.

"We say there is a principled reason that this extradition should not happen, this is an unprecedented prosecution," she told ABC Radio.

"He is in difficult prison conditions which are only going to get worse if he is extradited to the US and his life is at risk.

"It is enough. It has been over a decade and this case ought to be brought to an end."

The US extradition was rejected in January by a British judge due to the risk of depressed Assange taking his own life if placed in an American prison.

But in August, Assange lost a High Court battle to prevent the US government expanding the grounds for its appeal.

The 50-year-old remains in a London prison pending the court outcome.

His mental health continues to deteriorate along with his prolonged detention, Mr Shipton said.

The father-and-son have had only occasional 10-minute capped phone calls in recent times and throughout the height of COVID-19 restrictions in the UK, Assange was not allowed any physical contact with his own young children.

Ahead of Wednesday's hearing, Amnesty International called on the US to drop its charges against Assange, plus pushing for his immediate release from prison.

Amnesty's call was backed by a petition of more than 120,000 Australian signatures and a letter to Prime Minister Scott Morrison.

"These are people who are urging governments like Australia to prioritise the rights of a free press, and protect journalism around the world," Amnesty said in a statement.

"It is paramount that if Assange wishes, he be provided safe passage so he and his family can reunite safely in Australia."

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Australian Associated Press