The Australian is fighting extradition to the US to face 17 charges of violating the US Espionage Act and one of conspiring to commit computer intrusion.

The US added fresh allegations to his indictment last month, but Judge Vanessa Baraitser is yet to be formally served with the document which could affect Assange's extradition hearing in September.

Defence barrister Edward Fitzgerald believes the new indictment could delay the hearing until after the US election in November.

He suggested US Attorney-General William Barr may be using his client's case for "political reasons".

"We are concerned about a fresh request being made at this stage with the potential consequence of derailing proceedings and that the US attorney-general is doing this political reasons," Fitzgerald told the court.

"It's a fact that Mr Trump has described the defence case as a plot by the Democrats.

"I'm not sure if there's been some manipulation there."

Judge Baraitser asked Mr Fitzgerald to "reserve his comments" on the new indictment as it hadn't been served to her court yet.

Prosecution barrister Joel Smith said he could not commit to any timeline to serve the new indictment.

WikiLeaks editor Kristin Hrafnsson said there were no new charges being lodged and the development proved that the prosecution were unable to build a coherent and credible case.

"So they've scrapped their previous two indictments and gone for a third try," he said in a statement.

"They are wasting the court's time and flagrantly disregarding proper process."

Judge Baraitser warned the prosecution that the deadline had passed for the submission of any further evidence before the September extradition hearing, apart from psychiatric reports.

Assange appeared via videolink for the first time in more than three months. He was slouched in a chair wearing a beige sweater and pastel purple shirt.

He looked dejected, at one point sneezing into his hand. He wasn't wearing a face mask.

Judge Baraitser confirmed his extradition hearing would take place at the Central Criminal Court, known as the Old Bailey, on September 7.

The hearing has been set down for three weeks, but Fitzgerald said it would be prudent to make in a fourth week available.

Smith also said the court should wait and see if a fourth week was required.

The judge said she expected all members of the prosecution and defence counsels and Assange himself to attend the September hearing in person.

Assange is next due for a final call-over hearing in Westminster Magistrates' Court on August 14.

He is accused of publishing thousands of secret US diplomatic and military files, some of which revealed alleged war crimes in Iraq and Afghanistan, and of trying to recruit hackers to provide WikiLeaks with classified US information.

The charges carry a total sentence of 175 years' imprisonment.

Australian Associated Press