Ms Haylen has fended off criticism for weeks of her choice of former infrastructure executive and Labor staffer Josh Murray to lead the transport department.
A snap parliamentary inquiry into the appointment is hear evidence on Thursday and Friday.
But Ms Haylen will not appear, her office said on Wednesday.
"It's parliamentary convention that neither lower house MPs nor ministers appear in upper house inquiries, with the exception of budget estimates," a spokesman told AAP.
Mr Murray, a highly regarded former Laing O'Rourke executive and chief of staff to Labor premier Morris Iemma, began his tenure two weeks ago amid scandal.
Documents provided to parliament have shown recruiters aired concerns about his relevant experience and later interviewed him at the suggestion of Ms Haylen's staff.
Speaking notes prepared for the minister later revealed Mr Murray and his wife had donated $750 to the minister's election campaign, including by buying tickets to a fundraiser dinner.
Ms Haylen on Tuesday said it was "absurd" to connect the tickets with the later appointment, denied any wrongdoing and said her secretary was the right person for the job.
"I've been answering questions about this in the NSW parliament for some time now ... and from journalists," she told 2GB.
"I'm happy to answer those questions. I've got nothing to hide here."
Opposition transport spokeswoman Natalie Ward said it appeared Ms Haylen should have disclosed her relationship with Mr Murray as being more than that of an acquaintance.
But Premier Chris Minns backed Mr Murray and his minister, denying the ministerial code of conduct had been breached.
The inquiry beginning Thursday will also examine the pandemic-era appointment of Nationals federal secretary Emma Watts as NSW Cross-Border Assistant Commissioner.
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