Police have confirmed no one was inside a central Whanganui building - known to be sometimes occupied by squatters - when it was destroyed by fire in the early hours of Monday morning.

A police spokesperson said the investigation into the fire was ongoing and it continued to be treated as "unexplained".

"Police is liaising with Fire and Emergency New Zealand in the investigation."

The two-storey building on Victoria Ave was gutted from the inside leaving just a central brick wall and facade which was pulled down on Tuesday for safety reasons.

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The fire started just before 3am on Monday and produced a glow in the sky which firefighters could see as they left Whanganui Fire Station for the scene.

It was all hands on deck as every available firefighter in Whanganui was called in to help fight the blaze along with eight appliances, including one from Palmerston North.

Investigators are still working to determine the cause. Photo/ Bevan Conley
Investigators are still working to determine the cause. Photo/ Bevan Conley

The building, opposite Plymouth St on Victoria Ave, is owned by the Tupoho trust, has been unoccupied for several years and was already slated for demolition.

What remained of the building after Monday's fire. Photo/ Bevan Conley
What remained of the building after Monday's fire. Photo/ Bevan Conley

It was originally built in the late 1920s as a doctors' residence with two doctors living upstairs and surgeries downstairs.

The building was pulled down on Tuesday. Photo/ Simon Waters
The building was pulled down on Tuesday. Photo/ Simon Waters