Fire crews have stayed overnight at the scene of a suspicious fire on Blenheim's Wither Hills.
The 7ha blaze started in pine trees south of the Rotary Lookout at 11.45am yesterday, the fire service said.
Helicopters using monsoon buckets helped contain the fire by 4pm.
Authorities were treating the blaze as suspicious, southern fire communications shifts manager Rachel Butler said.
"Police were called and they've had a crew on overnight, just keeping a watch."