Julia Gillard's position as prime minister is "entirely untenable" following fresh revelations concerning her work as a lawyer almost 20 years ago, the opposition says.
It has been reported a newly released document confirms Ms Gillard told the West Australian corporate affairs commission that an incorporated association she helped set up in the early 1990s was not a trade union.
The so-called slush fund was allegedly later used by her boyfriend at the time, Bruce Wilson, to commit fraud, along with his fellow Australian Workers' Union official Ralph Blewitt.
Senior Liberal Christopher Pyne says Ms Gillard has to go after this week claiming she did not tell the AWU about the association because "the people I was dealing with were elected officials of the AWU".
"(But) the letter ... makes it entirely clear she was convincing the WA corporate affairs commission that it wasn't associated with the union," he told reporters in Canberra on Thursday.
"If the prime minister had any respect for the parliament and the Australian public or the Labor caucus she would resign as prime minister today."
Opposition Leader Tony Abbott told reporters: "If the prime minister has an explanation she better give it (and) she better give it quick smart."
Mr Abbott said Ms Gillard had been stonewalling but the fresh revelations suggest she may have broken the law.
"On the basis of the documentary evidence ... she gave false information to the West Australian authorities," he told the Nine Network.
"For a senior lawyer to make false claims to an important statutory body like this is a very, very serious matter ... it's in breach of the law I would think and it's certainly very, very unethical."
But parliamentary secretary Mark Dreyfus said Ms Gillard's leadership was "rock solid" and calls for her to resign were "ridiculous".
The prime minister simply told the corporate affairs commission the association wasn't a trade union "and that's absolutely correct", he told reporters.
"There's no smoking gun - there's not even a water pistol."
Fellow parliamentary secretary Richard Marles said everyone should take a deep breath.
"That is what a lawyer would do under instructions from their client," he said of Thursday's revelations.
Mr Marles insisted it was unremarkable Ms Gillard could not previously recall writing the letter because it took place almost two decades ago.
"(The opposition) think they've got a smoking bazooka ... but when they pull the trigger what they've got is a popgun," he told reporters.
Comment is being sought from the prime minister, who has consistently denied any wrongdoing.