National is questioning the integrity of an inquiry into the appointment of Deputy Police Commissioner, Wally Haumaha.

Haumaha's appointment is being investigated following revelations he stuck up for three of his colleagues accused of rape in 1993.

Nationals Chris Bishop says the opposition think there's a clear conflict of interest as NZ First's Tracey Martin will oversee the inquiry into Haumaha who has historic ties to NZ First.

"We do support some form of inquiry, the question is whether or not this is the appropriate inquiry given that the Minister who is in charge of the inquiry is a New Zealand First MP."

But the acting PM Winston Peters says Martin will adhere to her ministerial oath. 

"She's not a New Zealand First MP. She's a Cabinet Minister well aware of her responsibilities."

However, Nationals Tutehounuku  Korako says someone else should run the inquiry.     

"Not New Zealand First, I think there needs to be an independent person that actually leads it."

But Kelvin Davis is backing Martin because she's Internal Affairs Minister.

"The Minister of Internal Affairs is the right person to be overseeing the whole process and just need to let it run its course."

Minister Tracey Martin will soon name the investigator for this matter.