"Police searching the building have subsequently located a firearm hidden in the roof, which is believed to have been placed there by the offender," police said.
"The man is expected to face a number of charges and an update on these will be provided later today."
The Herald understands the man ran into a vehicle repair and servicing centre near the VTNZ - both properties were inside a semi-industrial part of Glenfield, close to Wairau Valley.
He ran into the building asking if he could use the toilet, a worker told the Herald.
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