Police Minister Michael Woodhouse is "not convinced" a loophole allowing the easy purchase of guns online has been revealed, after revelations a TV3 journalist managed to buy a rifle without a licence.

Police have launched a criminal investigation into the purchase of a gun online by MediaWorks journalist Heather du Plessis-Allan. She successfully purchased a .22 rifle using false details online.

An audit into online gun buying was leading to new processes which appeared to close the loophole that allowed the MediaWorks exploit, police said on Wednesday.

But Mr Woodhouse told TV3's Paul Henry programme he was not sure a loophole existed. "Putting aside the fact that someone has committed what I think is a pretty serious offence in order to make a point ... I'm not convinced there's some great loophole that has led to significant numbers of weapons being obtained illegally," he said.

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Police were already reviewing the Arms Act in advance of a possible change to the law in 2016, he said.

Statistics showed gun crime had reduced, despite the recent "cluster" of incidents and anecdotal stories, Mr Woodhouse said.

"Illegal weapons are serious, I'm not undermining that, but what we know is that weapons crime has reduced despite the cluster we had a couple of months ago. We watch that really closely.

"I'm still talking with ... the Police Association about what they're seeing and hearing on the front line but it's really difficult to write policy on anecdote."

He did not want to comment on whether du Plessis-Allan's sting was good or bad journalism because it was now a police investigation.

TV3's Story broadcast about the gun purchase on Wednesday night after co-host du Plessis-Allan told Radio Live the .22 rifle had been bought for $300 using a fake name and fake gun licence. "It wasn't difficult for me to do this. I didn't have to make fake IDs or anything."

She then outlined the process she went through. "I used a fake name. I used a fake gun licence." She said she filled it out and sent it in.

It was worrying she was able to buy a gun "under the name of someone who doesn't exist", she said.

Radio Live's Duncan Garner, who interviewed du Plessis-Allen, said the rifle had been bought to test the law and there was a strong "public interest" defence.

Guncity owner David Tipple has since pledged to privately prosecute du Plessis-Allan and leave her with a criminal conviction, if the police investigation falls through.

"I'm ready for the battle," he told the New Zealand Herald.

"She's going down 100 per cent. They're going down. What so many people don't remember is even if the cops say we won't prosecute her, then I can prosecute her and I will. She will get a criminal record because of what she has done."

Mr Tipple said he was "so upset" du Plessis-Allan used false details to order and collect a rifle from his store.

A spokeswoman for MediaWorks said: "We believe it is in the public interest that this serious loophole in the gun laws is closed. We support this story and the Story journalists completely."

Inspector Peter Gibson said an investigation had been launched after an allegation emerged false details had been used to fraudulently obtain a firearm via an online dealer.

Possessing a firearm without a licence was a criminal offence and could lead to three months in prison or $1000 fine.

The charge for "obtaining by deception" carried penalties of up to seven years in prison.

"Police take any incident involving the illegal obtaining or possessing of firearms extremely seriously," Mr Gibson said.

A review of firearms licensing was already underway and it included an audit of the country's largest online retailer, police said.

The audit had revealed no problems with online sales but a wider audit of all major online gun dealers was being carried out, it said.

Police were also updating its systems for buying guns online, meaning those doing so would have to go to a police station and show the police arms officer the firearms licence before any online purchase was approved.

Du Plessis-Allan told the Herald the story was done because there was a need to change the system and that had already happened.

False details used to "trick" staff

Mr Tipple said police had a "public interest" clause which stopped a prosecution. But he said the response on social media since the revelation of the gun purchase showed many people believed his company - which bills itself as the "world's largest gun store" - was in the wrong.

"I can't afford half the people looking at this and thinking we did something wrong. People get the idea that the only one in New Zealand who can do a criminal prosecution is police. I've done it once before, taken a private prosecution. It cost them a fortune. It cost me a grand."

Mr Tipple said the stunt saw false details used to trick his staff - and it was done by inventing the details of a police officer who was meant to carry out the safety checks. It was a case of any system being able to be fooled if the person doing so was intent on it, he said.

He said anyone who tried a similar stunt would be caught when the police officer's details manufactured on the form were checked. At that point, he said it would have seen the Armed Offenders Squad sent to the house where the rifle was delivered.

Mr Tipple said du Plessis-Allan had also been lucky when the firearms licence number used was confirmed as valid when the shop staff checked with police before sending the rifle out.

"She made up the number but she hit the jackpot." Staff were able to check the number by the system used by police meant gun shops weren't able to check the name matched up, he said.

He said the consequence of it was to give people the false impression there was a problem with illegally purchased guns on New Zealand streets.

- Additional reporting: NZME