The smouldering wreck of a warehouse in the Auckland suburb of Penrose is being demolished this afternoon.

The Humes Pipeline Systems building has been written off after a huge fire which broke out last night.

Goodman owns the building and CEO John Dakin said demolition work will continue overnight.

"Humes are supportive of going ahead with demolishing the the building and we're working with them to see if we can find them some alternative accommodation," he said.

Fire services responded to the blaze at the Great South Road property last night around 9PM and had it under control within the hour.

Dakin said there are many reasons why the building has to come down now.

"It is a bit of a hazard in terms of its boundary to the railway line, so that's probably the most important thing to remove that, and also from a health and safety point of view."

The fire forced trains on the Onehunga line to cease operations overnight due to fears the building could collapse onto the tracks.

Only one line was running when the service started again this morning.

Investigations are still underway into what caused the fire.