Centre Alliance senator Rex Patrick has declared the former defence minister Christopher Pyne “cannot un-know what he knows” ahead of a Senate hearing on Thursday to test compliance with the prime minister’s statement of ministerial standards.

Thursday’s hearing in Canberra will see Pyne and the former foreign affairs minister Julie Bishop give evidence to the Senate’s finance and public administration references committee via teleconference. The inquiry has been triggered by controversy about Pyne and Bishop’s private-sector appointments post-politics in fields overlapping directly with their portfolio experience.

Pyne has joined consulting firm EY to help build its defence business, and Bishop has joined the board of Palladium, one of the government’s biggest private aid contractors. Thursday’s hearing will also see evidence given by executives of the two firms.

'Pub test' gets Pythonesque as Martin Parkinson grilled over Pyne and Bishop's jobs Read more

The current federal ministerial standards require that ministers do not lobby, advocate for or have business meetings with government, parliamentarians or the defence force on “any matters on which they have had official dealings” in the past 18 months. The standards also say that ministers should not use information they have obtained in office for private gain.

Both Pyne and Bishop insist they are fully aware of their responsibilities, and will comply with the standards, in their new roles.

Scott Morrison also asked his outgoing departmental head, Martin Parkinson, to make inquiries about the appointments. Parkinson spoke to the former ministers and found in July “no grounds to believe that either Mr Pyne of Ms Bishop [had] breached the standards”.

Parkinson fronted the Senate committee last week and said he had found no cause for concern based on the accounts of Pyne and Bishop. Having been asked whether the appointments passed “the pub test”, Parkinson told senators he was not required to administer the “pub test”, he was required to administer a document formulated by the prime minister.

He observed the ministerial standards had remained constant under Liberal and Labor prime ministers. He also noted he had “no investigative powers” and “no legislative backing” to pursue breaches.

Patrick told Guardian Australia the purpose of the current Senate inquiry was to lead to recommendations that the ministerial standards should be strengthened. He said Pyne needed to explain how he would navigate his new role while remaining compliant with the standards.

“[Pyne] cannot un-know what he knows,” Patrick said. “He’s been briefed on matters related to problems inside defence, future projects, on what commercial companies are offering.

“When he forms up his advice to EY, when they say to him ‘where do you think we should invest our money?’, he is faced with a dilemma.

“EY will expect him to give proper advice. Either way there’s a conflict. It’s hard to form proper advice without breaching the standard”.

In a written submission to the committee, Bishop argued serving MPs and officials should be held responsible for not meeting with former ministers. The long-serving foreign affairs minister contended the ministerial code of conduct would be “more workable and enforceable” if it shifted the onus to former colleagues still accountable to parliament.

“The onus could and arguably should be placed upon current serving officials to not hold meetings with former ministers for the period of 18 months after those ministers cease to hold office,” Bishop said in her submission.

“Current ministers and government officials are subject to ongoing scrutiny through the parliament, including question time and Senate estimates, and by the media. There is obvious redress available with regard to current serving ministers and officials, should they breach this protocol.”