Premier Chris Minns has denied the state opposition's claim that the transport minister breached the ministerial code of conduct, amid revelations a Labor-linked public servant in her office may have engaged in political work.

An investigation has been launched into Transport for NSW bureaucrat Kieren Ash, who was seconded to work in Ms Haylen's office in a non-political position known as a department liaison officer (DLO).

Mr Ash, who ran as Labor's candidate for Pittwater in 2015 and managed the party's unsuccessful campaign to win Balmain this year, was requested specifically by Ms Haylen's office.

A DLO's job is to act as an impartial conduit between the department and the minister's office, avoiding party political activities.

Documents tabled to parliament on Friday revealed Mr Ash was involved in work that could be seen as political, including helping to craft speeches with a "positive tone" about the government, and a "negative tone" about the opposition.

Mr Ash, far left wearing glasses, was seconded to work in Jo Haylen's office.(Supplied)

The documents also showed he agreed to help set up a barbecue celebrating Labor's election victory.

On Monday, the premier admitted organising such an event fell outside the remit of a DLO.

He said he did not have an answer as to whether Ms Haylen knew everything Mr Ash was up to.

"I don't think it would be reasonable to expect that she'd know what reasonably junior members of her staff were doing 24/7, or even during work hours," Mr Minns said.

The opposition has accused the minister of breaching the ministerial code of conduct.

The Coalition pointed to the section of the code that states ministers "have a responsibility to ensure that they do not act in a way that would place others, including public servants, in a position that would require them to breach the law or their own ethical obligations."

Opposition transport spokesperson Natalie Ward said: "You cannot be influencing a member of the bureaucracy or a public servant to undertake political work or to breach their ethical obligations."

'Could have been done better'

The premier said that DLOs had been appointed by the previous Coalition government.(AAP: Bianca De Marchi)

Ms Haylen said on Sunday the documents were first brought to her attention on Friday, but she refused to clarify if she had any prior knowledge of the work in question.

She has referred the documents to Mr Ash's employer, Transport for NSW, to investigate.

While the premier admitted "things could have been done better", he reiterated he did not want Ms Haylen to resign, adding: "I don't believe that there's been a breach of the ministerial code."

He also said he did not believe there was a widespread problem regarding the use of DLOs in his government.

He pointed out that while public servants such as Mr Ash had Labor affiliations, they were appointed by the previous government.

"In retrospect, we should have taken the advice from the secretary of transport [on]… who was best placed to serve in that role.

"Lesson learned, and we move on."

Ms Haylen said on Sunday she would not quit, even if there were adverse findings from the investigation into Mr Ash.

She is set to face questions on the matter at a budget estimates hearing on Tuesday.