National says some police officers are too afraid for their future prospects to give evidence to the inquiry into the appointment process of Deputy Commissioner Wally Haumaha.
It says some form of protection for witnesses should be offered by inquiry head Mary Scholtens QC, who starts next Monday.
In the course of a day, National shifted from saying Haumaha should not resign to saying he should stand aside during the inquiry in light of new Herald revelations about Haumaha contacting a witness to an alleged incident of bullying.
Former Police Minister Paula Bennett said one man, Haumaha, was putting at risk the good work the Police had done over the past 11 years to change its culture.
She said it should be widened to look at his suitability.
And Prime Minister Jacinda Ardern made it clear that if the inquiry showed up major deficiencies in the appointment process, the Deputy Commissioners' job could be re-opened for appointment.
Scholtens was appointed last Friday after a delay caused by the resignation of the first inquirer, Pauline Kingi, owing to a perceived conflict of interest.
The inquiry was originally to look at whether all relevant information was properly provided to or gathered by the State Services Commission panel.
It was commissioned after revelations of dismissive comments Haumaha made in 2004 in response to allegations that three former colleagues had raped Louise Nicholas. But it may also assess whether allegations of bullying behavior in a 2016 cross-agency team were known to the appointment panel.
National police spokesman Chris Bishop said he was aware of a number of officers who wanted to give evidence to the inquiry but were concerned about their identities being kept confidential for fear of possible consequences down the track after the inquiry reported back.
"In order to get to the truth, the inquiry must institute some form of protected disclosure regime, and do it quickly," Bishop said.
New Zealand First Minister Shane Jones called on the police leadership to protect its information from leaks.
"There's a mare's nest in the police," he said. "My message to the leadership of the Police which are meant to be independent of politics is guard your information a lot better because at the end of the day people trust that the Police will carry out their role and place a great deal of accent on the security of information ..... the fact that it is dribbling out into the public domain does not reflect well on the Police at all."
Bennett, National's deputy leader, opened up a new front for National on the Haumaha issue, concentrating on the women's aspect and saying his suitability needed to be reviewed.
"I just advocate for the Police with everything I've got," she said. "They've worked really hard over the last 11 years in particular since the Bazley report [Commission of Inquiry into Police Conduct] to change their culture and be more open to victims of sexual assault and intimidation.
"What upsets me most at the moment is we are seeing that culture go backward potentially because of one man."
National leader Simon Bridges did not call for Haumaha's resignation earlier in the day, merely saying his position was "perilous" but by the afternoon Paula Bennett and Chris Bishop were calling on him to stand aside.
Bridges said the terms of reference of the inquiry should be broad enough to cover conflicts of interests and said Haumaha's association with New Zealand First should have been declared.
Deputy Prime Minister Winston Peters reacted angrily to the Herald story about Haumaha contacting a witness, saying it was "utter unadulterated drivel."
"I've got legal information," Peters said when asked on what basis he made that claim.
He said he had not been in touch with Haumaha but he declined to say whether or not he was a friend of his.
He reiterated his statement that the reason he had spoken at a marae celebration last year after Haumaha's promotion to Assistant Commissioner was because the Police had asked him.