Police Minister Stuart Nash has admitted he didn't read official advice on options for phasing in 1800 new police officers over five years.

"I didn't read any paper that said phasing in over five years. For me, phasing in over five years was just not an option I was prepared to consider," Nash told a parliamentary committee today.

The Government says it will deliver 1800 new officers over three years. There are concerns that will put more pressure on the prison system.

Nash was quizzed on the advice from police by National's Chris Bishop.

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"I don't read papers like that because there is a coalition promise that I will work to deliver. Any paper, any suggestion, that we are not going to meet our coalition deal of 1800 police over three years, certainly one that suggests its going to take five years, I'm just not even interested in seeing," Nash said.

"You are kidding? Are you seriously saying to the committee that you received a paper about phasing options for the coalition commitment that you are talking about and you didn't read it?" Bishop asked.

"Not even interested," Nash responded.

Nash took a paper to Cabinet which outlined options for phasing in the extra police over three years and four years, with his preference being three years.

A new prison announced yesterday by the Government will not be ready until 2022 and Corrections is building pop-up prisons and using double-bunking to accommodate the growing muster.

Bishop pointed out that Nash's own Cabinet paper contained advice from the Justice Ministry and Corrections Department that between 600 and 900 more prison beds would be needed if police numbers were boosted.

Nash said he had not ignored that advice but preferred police advice over that of Justice and Corrections.

"We get advice from all over the place … you have to make a decision on whether you take that advice or whether you take other advice.

"On the balance of probabilities I've taken police advice over Justice and Corrections advice."

Nash said he "absolutely believed" that more police would reduce crime and the number of people in prison.

His Cabinet paper seeking funding for the new police officers says that "severe pressure on Corrections facilities means there is little tolerance for even minor changes in demand for prison places without additional funding".

Speaking after the committee, Commissioner Mike Bush said he was determined that the extra police would not blow out the prison population but could not guarantee it.

"Extra resource means prevention, so there will be less crime.

"We showed back in, I think around 2014, that an extra investment in police did not equate to a rise in the prison population."

Bishop told reporters afterwards that Nash's admission that he hadn't read official advice was "astonishing".

"Frankly, I think people expect better of ministers.

"I just find it astonishing that he admitted to a parliamentary select committee he hasn't even read a paper provided by his agency about his flagship policy."

He said the Government should swallow its pride and build a prison big enough to take the extra prisoners that would come as a result of more police.

"What you've got is branches of government working at completely different odds with each other. You've got the Police Minister saying 'let's smash the gangs, let's put 1800 new police on the beat and lock up more criminals' and you've got the Corrections Minister and the Department of Corrections only putting 74 new beds in place to cope with that," Bishop said.