Amy Brown, the Investment NSW chief executive and secretary of the Department of Enterprise, Investment and Trade, told a parliamentary inquiry she had reservations about appointing the former deputy premier.

"I was nervous about it because he had some history with the NSW government that may make it difficult for him to take up the role without media and public controversy," Ms Brown said on Wednesday.

She was influenced "to a degree" when cabinet changed the hiring process to a ministerial appointment, and did not want to choose a candidate who would be replaced at the whim of the government.

"It did not amount to undue influence because at all times I felt that the decision was mine," Ms Brown said.

She dismissed the suggestion she preferred Mr Barilaro as the candidate, but said she did not want to be the only one supporting his appointment.

"There was a high degree of interest in the outcome of the process, and I didn't want to necessarily stand behind it alone."

Ms Brown told the inquiry she approached Department of Premier and Cabinet secretary Michael Coutts-Trotter to inform him of Mr Barilaro's application and eventual selection.

"I said something along the lines of 'Mr Barilaro is the successful candidate ... I'm nervous about it, I wonder if you might want to check in with the premier'," she said.

The DPC secretary told her to leave the issue with him.

"He didn't say what he was going to do necessarily," she said, noting she also did not receive a substantial answer.

Her explanation for changes to a report by a recruitment firm increasing Mr Barilaro's rating was that the original report was inaccurate, unsolicited, and did not reflect concerns from the panel about another candidate.

A final version of the report upgraded Mr Barilaro to the most desirable candidate when his application was updated with reference checks, Ms Brown said.

However, Ms Brown could not confirm whether the final version of the report was sent to the panel members before he was appointed to the role.

Mr Barilaro's referees included former Liberal MP Arthur Sinodinos, and Department of Regional NSW secretary Gary Barnes.

Ms Brown said she was not concerned panel members would reject Mr Barilaro when they received the report.

"We'd had multiple conversations that they were satisfied with Mr Barilaro ... so I was not expecting anyone to not sign the report," she said.

"It was a matter of the paperwork catching up with the reality."

Earlier, Premier Dominic Perrottet announced Stuart Ayres had quit the ministry and the Liberal deputy leader role, following a draft of the government's review into the appointment.

"The issues in the review go directly to the engagement of Mr Ayres with a department secretary in respect to the recruitment process," the premier said.

"It raises questions in respect to the ministerial code of conduct."

Mr Ayres has denied any wrongdoing but welcomed an investigation.

"The review clearly demonstrates that the process was not at arm's length," Mr Perrottet said.

Ms Brown agreed.

"He (Mr Ayres) was not at arm's length from the process, there were multiple intersection points," she said.

Mr Ayres also sent a copy of the job ad to Mr Barilaro after he texted him expressing an interest in the role.

"He (Mr Ayres) said 'John Barilaro's interested in applying, I've forwarded him the job ad like I would anyone else'," Ms Brown said.

Many others had expressed an interest in the role, however Mr Ayres said on Tuesday he did not send anyone else the job ad.

Mr Barilaro stood down from the job less than two weeks after his appointment was announced in June, saying it had become untenable.

Australian Associated Press