Gabriel Shipton says he knew for weeks that his brother Julian Assange was about to be released from the UK's high-security Belmarsh Prison and that the WikiLeaks founder was both "excited and anxious" about his looming freedom.

"I've been chatting to him over the past weeks about the finalisation of the little bits and pieces that all needed to come together," Mr Shipton told 7.30.

"Julian has been preparing to get on this flight and ...  he's been very excited, a little bit anxious, mostly excited about being free after all these years.

"It's over 13 years in one form of detention or another in the United Kingdom."

Mr Shipton said the last time he had seen his brother was while he was in prison, and to see footage of him boarding the plane was heartening.

"He looked quite sprightly going up those stairs," Mr Shipton told 7.30."The last time I saw him was in the visitors' room inside Belmarsh, so it's great to see him in the in the daylight."

However, Mr Shipton did not want to celebrate his brother's impending return too early.

It was revealed earlier on Tuesday that Assange had made a plea deal with the US Department of Justice (DOJ).

Before he makes it back to Australia, Assange still has to face a US judge in the Northern Mariana Islands on Wednesday morning, the closest US jurisdiction to Australia.

There he is expected to plead guilty to violating US espionage law, which if accepted will allow him to return to Australia.

The long fight

Mr Shipton said he believed Assange was travelling with members of the Australian government, as the years-long campaign to free him was hopefully drawing to a close. extradition to Sweden over a rape allegation, he then spent five years in Belmarsh after the embassy expelled him over his behaviour.

He was carried out of the embassy by police and jailed before potentially facing extradition to the US.

While the plea deal has been done with the DOJ, some Americans are still unhappy Assange, could be freed, former US vice-president Mike Pence among them.

"Julian Assange endangered the lives of our troops in a time of war and should have been prosecuted to the fullest extent of the law," Mr Pence tweeted on Tuesday.

"The Biden administration's plea deal with Assange is a miscarriage of justice and dishonors the service and sacrifice of the men and women of our Armed Forces and their families. 

"There should be no plea deals to avoid prison for anyone that endangers the security of our military or the national security of the United States. Ever."

While Mr Shipton remains cautious, he thanked world leaders for rallying around the cause to free his brother, especially Ambassador to the US Kevin Rudd and UK High Commissioner Stephen Smith.

'Monumental': Shipton thanks government

He said the Australian government played a key role in putting pressure on others to release Assange ahead of an appeal in early July, in which Mr Shipton felt the DOJ might face an embarrassing trial in its efforts to extradite Assange.

"I don't believe the US DOJ really wanted to have an appeal around these freedom of expression grounds being heard in the UK court," he said.

"I think the support from the Australian government has been monumental. 

"The resolution that passed the House of Representatives in February really changed the tone of the UK courts."

That motion was to urge the US and UK to allow Assange to return to Australia.

"I believe that the actions by the Australian parliament and by the Australian government and the Australian people really led to this position," Mr Shipton said.

"If Julian didn't have that support of this government, then they wouldn't be able to put this deal together and have him returning to Australia to be a free man."

Asked what he would do with his brother upon a return to Australia, Mr Shipton gave a typically Australian answer.

"Everybody's so relieved and so happy that this is finally happening," he told 7.30.

"It's been a bloody long slog for Julian's family and everyone who's been out there supporting him ... and I think we're all looking forward to sitting back and having a beer or a champagne and celebrating him being back in Australia."

Julian Assange and WikiLeaks: A timeline

2006: Julian Assange starts WikiLeaks

2010: WikiLeaks angers the US by releasing huge caches of classified documents

Dec 2010: Assange is arrested in the UK over a rape allegation in Sweden, which he denies

Feb 2011: A court orders his extradition to Sweden. The appeal attempts fail

Jun 2012: Assange takes refuge in Ecuador's London embassy

Apr 2019: Ecuador revokes his asylum and, citing "spoiled brat" behaviour, evicts him. He's arrested at the embassy

Jun 2019: The US requests his extradition to face espionage charges

Feb 2020: Extradition hearings begin in a London court, but are delayed by COVID-19

Jan 2021: A London judge decides not to extradite Assange due to mental health concerns

Jun 2022: After a higher court overrules that decision, the UK government orders his extradition to the US

Jun 2023: The UK High Court rejects his appeal

Feb 2024: The court holds another hearing to consider whether to allow Assange to appeal again.

May 2024: The court rules Assange can appeal again, staving off his extradition for now.