A suspicious blaze that sent flames 20 metres high south of Swannanoa yesterday is to be investigated by police.

Waimakariri principal rural fire officer Tim Sheppard, who investigated the scrub fire, said the matter had been passed on to police.

Sheppard said the fire off Two Chain Rd appeared to be deliberately lit.

School pupils were among the suspects, he said.

"School is out," he said. "Fires don't start by themselves."

Strong winds blew the fire down a shingle track towards the Waimakariri River.

The fire burnt about a hectare of scrub before crews from Rangiora, Cust, Swannanoa and Pines Beach brought it under control.

- © Fairfax NZ News