A UK court held hearings on Tuesday and Wednesday to consider a US extradition request for Assange. Judges Victoria Sharp and Jeremy Johnson said they would issue their ruling later.
Shipton talked to his son on the phone on Wednesday following a rally outside the High Court in London, during which the investigative journalist's supporters urged the UK authorities to release Assange and not to extradite him to Washington, Australian broadcaster ABC reported. After that, Assange's father appealed to the Australian government.
"You have to show the United Kingdom and United States that you're serious," he was quoted as saying by ABC.
Shipton also suggested that Canberra threaten the UK and the US with restricting access to their consular buildings in Australia to demonstrate the seriousness of its intention to have Assange released.
The journalist's father added that his son sounded "a bit throaty" on the phone as he had a cold and that he had been "under a considerable amount of stress" lately.
"He is the son of the Australian soil and acknowledged to be one of the great journalists in Australia, in the world ... We require the United Kingdom, as a friendly ally … to take some sincere intentions to return Julian home to Australia," ABC quoted Shipton as saying.
WikiLeaks was founded by Assange in 2006 but rose to prominence in 2010 when it began publishing large-scale leaks of classified government information, including from the US.