Police believe they know how 24 year old Matu Reid obtained a firearm before shooting his co-workers in Auckland CBD last year

Reid went on a deadly rampage with a pump action shotgun at a building site on Queen Street in Auckland Central last July.

He fatally shot two co-workers and injured 10 others; and died in suspected suicide.

Detectives allege the shotgun was purchased legally from a retail store nine months beforehand in New Zealand; before it was sold onto the black market.

It's a tactic known as using a straw buyer, or retail diversion; believed to be the most common way for firearms to end up in criminal hands.