A fire that damaged a large tent used for concerts and other events at Mt Smart Stadium in Auckland is being treated as arson.

Firefighters last night battled to save the huge circus-type structure from being destroyed.

Fire safety officer Ray Coleman said the incident was being treated as suspicious and police had been called in.

"The structure was moderately damaged. The fire was confined to the south west area amongst some tarpaulins.

"There was just a bit of damage to an area inside the tent. The structure is still standing," he said.

Mr Coleman said it did not know at this stage what was used to start the fire.

"There was nobody in the big top when the fire started last night. That area is completely isolated from the stadium."

Mr Coleman said he hadn't noticed any lights on or any activity in the stadium last night.

The Fire Service's investigations had been completed and the matter had now been passed over to the police.

The blaze began among rolls of PVC under the stage area, creating flames and dense smoke, and firefighters had to use breathing apparatus.