Police have fended off criticism that the collection of now-banned firearms is failing and more could be done to get an accurate picture of how many such guns there are.

Deputy Commissioner Mike Clement clashed with National MPs during an at times terse select committee appearance this morning over the Government buyback scheme and the efforts to collect prohibited firearms.

The scheme and six-month amnesty was put in place after gun law reforms - supported by all parties but Act - banned most military-style semi-automatic (MSSA) firearms in the aftermath of the March 15 terrorist attack.

Clement updated the committee on progress nearing the halfway stage of the amnesty: 12,621 people have handed in 19,837 firearms and 73,949 parts; $36.7 million has been paid out.

Of the 14,000 or so MSSAs that are registered with police, 2500 have been handed in.

Clement cited estimates that put the number of now-banned firearms between 56,000 and 173,000.

But National MP Brett Hudson said there were police estimates of 240,000, a figure that Clement did not dispute, though he added that the true number was simply unknown.

With less than 20,000 firearms collected nearing the mid-way point of the amnesty period, National MP Nick Smith said: "We are a long way from making us safer."

"How on Earth will any one of us be able to stand up in front of the public of New Zealand and say 'you are safe' if we believe there are tens of thousands of these things still out there?"

Clement said police were as keen to get as many MSSAs off the streets as anyone.

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Hudson noted in a report from consultancy group KPMG that the buyback option chosen was the one with the lowest expected compliance.

An option with higher expected compliance would have been to have one price for a model of firearm, regardless of its condition.

Clement did not dispute this, but said it was a political decision.

"These are choices put to the Government and choices were made. That's not for me.

"I've seen in a practical sense the process working very well."

Hudson and Smith said there had been a lack of effort by police, such as looking at records of imported firearms, to get a more accurate picture of the number of now-prohibited firearms.

But Clement said that such detailed information was not available.

"We just simply haven't captured the detail in the way you're suggesting that would enable us to point to the size of the fleet."

He did not overtly criticise National, but said that Parliament had almost unanimously supported the gun law reforms, but "I'm not hearing the entire Parliament supporting this process".

He accepted Smith's and Hudson's challenge and said police would look to what it could do to get a more accurate picture of the number of MSSAs.

He said the journey to get these firearms off the streets would take years and possibly more amnesty periods.

Independent surveying had shown between 92 to 96 per cent of gun owners had had a positive experience at the collection events, he said.

"I can count the people who are still cross on one hand.

"I've spoken to dozens of people ... but I cannot find somebody who is cross about the value they got for their firearms."

But Hudson said that the gun owners who are going to police collection events were not the ones who were unhappy with the scheme.

After the hearing, Clement said it was fair for National MPs to ask police about the number of MSSAs, even though successive Parliaments failed to legislate for a national gun register - something the current Government is now proposing.

He said police would use discretion if they found people with illegal firearms after the amnesty expired on December 20, but they should not expect a police warning.

"Why would you take that risk? Please don't be the person that looks back on the 21st of December and says, 'Gee, I missed that opportunity and now I'm unlawfully in possession of a firearm.'

"Bring your guns in."