A fire in the toilets of the Duncan Pavilion on Monday afternoon is being treated as suspicious.

Whanganui station officer Aaron Summerhays said firefighters were called to extinguish the blaze, which had started in a plastic toilet roll holder just after 5.30pm.

The toilet block, on the bottom level of the building, was filled with smoke and firefighters required breathing apparatus to go inside. They were there for about 20 minutes and there was no other significant damage to the block.

Mr Summerhays said police would investigate the blaze. It was similar to another suspicious fire at the Gonville Domain toilets in January when a hand-drying unit was deliberately lit, causing damage to the block.