A fire at a Ngaruawahia school which burnt the deck of a classroom is being treated as suspicious.

Two fire crews attended the fire at Waipa Primary School on Kia Toa St at 10.23am this morning.

Northern Fire Communications shift manager Jaron Philips said the fire had been extinguished by the caretaker and was smouldering when firefighters arrived.

The fire had burnt through the deck of a classroom, he said.

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Police were on the scene and a specialist fire investigator was looking into the cause of the fire.