Labour has demanded an inquiry into online gun sales, following revelations the man behind Northland's double fatal shooting was able to buy firearms online.

But the police minister says the police need to be left to carry out their investigation.

The Herald on Sunday revealed today that Quinn Patterson - who killed two property inspectors in Whangarei on Wednesday - was able to buy weapons on Trade Me, despite not having a firearms licence. He was also selling gun accessories online.

Read more: Patterson's Trade Me gun deals

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Police have not said whether they asked to see his collection of amour after being called to the property to inspect a structure Patterson said he used for "target practice" about a month ago.

Labour's police spokesman Stuart Nash said the Government should launch an independent inquiry into online sales.

"Someone independent who can look at what went on, why it did and why this chap could buy guns so easily, and flout the system.

"I am concerned about guns being sold on the internet."

He still believed firearms should be allowed to be sold on Trade Me but there had to be a "much more robust process".

"That could include both parties meeting at a police station for the transaction, or go through a licenced gun dealer.

"This put all gun owners in a bad light," Nash said.

All online guns sales should be suspended, until there's been an inquiry, Nash said.

But police minister Paula Bennett said "Labour are all over the place when it comes to laws and rules around firearms".

"Police are investigating how this guy got hold of guns.

"They certainly turned him down for a licence. I think there are unanswered questions but I think that we should all make sure that police are given the space to do a proper investigation. If there is a loophole then we will close it."

"The calls from Labour are them barking at every car. We don't know all the facts."

She said the loophole had been fixed after NewstalkZB radio host Heath du Plessis-Allan was able to buy guns online.

"Those who want to buy a gun online have to go into a police station and show their licence and identification.

"To date, what I've seen is it is enough. But I want police to do their investigation thoroughly."

Patterson shot dead mother and daughter Wendy and Natanya Campbell this week. He shot contractor Jeff Pipe, who had gone with them. Pipe is recovering at home.

A former police negotiator, Lance Burdett, told du Plessis-Allan on NewstalkZB this morning he was surprised at how easily Patterson was able to buy weapons on Trade Me.

But he didn't want to see everyone persecuted over it.

"Should we licence firearms or people, or should we do both?

"There will always be a criminal element. And we shouldn't target everybody with that same brush and punish the whole country over one or two incidents.

"I'm sure there is a better way than what we currently do. It is very easy to get firearms into the country it would seem."

Du Plessis-Allan said Patterson displayed several warning signs and asked how quickly police would have realised what they were dealing with.

"They are very dynamic these situations," Burdett said.

"Obviously if there has been a shooting, than there are firearms. How many firearms do they have? You find out over due course and that's the kind of thing you concentrate on as a negotiator, try and find out what's gone on.

"It's so easy to go back over these events and say 'We should have done this, we should have done that'. Because we can join the dots after the event."

He said police were caught between a "rock and a hard place" when dealing with potential criminals in terms of the police and Government becoming "overbearing".

"There are some generalisations we can make. The person is isolated... usually they are in a very remote location. They often wear camouflage clothing, round the streets all the time. that's very, very common with these types of shootings.

"I have to say that no normal person would wear camouflage clothing down Queen St for example unless they are military or going hunting later that day.

"However do you want to persecute everybody based on generalisations? Possibly no."

He said it was "the lack of human contact" that caused shootings.

"How may of us know our next door neighbour? We become isolated like that, we start to feel the rest of the world is against us.

"The more isolated we physically are, the more isolated mentally we become.

"Socialisation of one of the best things you can do for yourself."

He said in shootings across the US, isolated people were responsible. He studied radicalisation as part of his Masters degree.

"It was always the lone wolf. ISIS target those people. Gangs in the 70s and 80s were formed because isolated people joined them."

Burdett said the case brought back memories of gunman Jan Molenaar, who shot dead police officer Len Snee and injured two other officers from his home in Napier in 2009.

He was the lead negotiator on the case.

"They are very, very similar aren't they? They are basically a lone person who is isolated from the world and has some sort of fixation with firearms.

"They may or may not have come into contact with police over previous occasions but most times go under the radar. Yes, a very, very similar event."

Snee's body lay at the scene for a day before police approached the house, but in the Northland case, police entered immediately and tried to offer assistance.

"A lot depends on where the unfortunate people lay," Burdett said.

"So if it was right outside the premises, I'd hazard a guess, they wouldn't have done anything different.

"Every incident varies depending on the location... They would have deemed it safe to go in an render first aid."

He said he did not think there was an increase in incidents of people holed up in homes with a cache of guns, but the public had a tendency to remember the negative events.

There were more assaults however.

"You talked about Jan Molenaar, how long ago was that? But it's still fresh in your mind.

"We are finding people are being confronted more and more than previously in the five years."