Newshub can reveal Northland gunman Quinn Patterson had help acquiring his illegal stash of firearms.
Patterson didn't have a firearms licence but used a valid licence number that belonged to someone else.
Some of the guns, or parts of guns, were purchased on Trade Me, while others, it's believed, were imported from overseas.
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"There are a number of areas where the process could have fallen down in this particular instance," Trade Me's Trust and Safety manager Jon Duffy told Newshub.
"We are not sure where it has so we don't want to comment further until we've got all the facts."
Newshub has revealed Patterson's friend saw shotguns, pistols, grenades and bulletproof vests at the property in the weeks before Wendy and Natanya Campbell were killed.
Police say he never had a gun licence.
But here's the problem. Patterson's online Trade Me profile was linked to a valid gun licence number - a gun licence belonging to another user.
So could two different users buy and sell guns on a single licence?
"It is possible. We do run checks but it is possible that sometimes that sneaks through the system," Mr Duffy said.
Patterson's friend, Ross Thompson, said as well as Trade Me, Patterson also imported firearms using a variety of identities and addresses.
"To get weapons he had different names, because he couldn't use his name he reckons," he told Newshub.
"Trade Me, anything, but he didn't use his own place; otherwise the police would come and have a look at it."
Police visited Patterson's place at Mt Tiger Rd a month ago, after receiving a complaint about a homemade platform Patterson built for target practice.
On Saturday, police said the officer who visited decided the platform he'd built was a tenancy matter - not a police one - and left it at that.
But, in another twist, Newshub had been told police later executed a search warrant at Patterson's property to look for firearms. But police wouldn't confirm this, only saying they can't answer specific questions that may be relevant to their investigation.