A bach has burned to the ground at Ahipara in the area's sixth suspicious fire since Christmas.
The latest suspected arson occurred west of Kōhanga/Shipwreck Bay on Tuesday in a bay accessible only at low tide.
A two-bedroom bach with an attached house bus was destroyed, and the flames spread into about half a hectare of grass and marshland.
No one was home at the time.
Fire and Emergency NZ volunteer support officer Brett Southey said the alarm was raised by someone who saw a red glow in the sky about 5.25am.
It was high tide so no one could reach the fire at that time.
After a second call about 8.20am a Salt Air helicopter from Paihia toput most of the fire out by scooping water from the sea with a monsoon bucket.
Although the Ahipara Fire Brigade truck is four-wheel-drive it still couldn't get through the tide and soft sand, so firefighters went in a ute with a portable pump and hand tools.
They finished putting the fire out by pumping water from a rock pool.
Southey said the chopper extinguished most of the blaze before it could endanger other baches.
Detectives from the Kaitaia police and fire investigator Gary Beer attended.
Beer said the fire was suspicious and police were investigating further.
Earlier fires in Ahipara include one that destroyed a two-storey home in Kaka St early on February 11 and a blaze that swept through 56ha of scrub and native bush on a ridge above Foreshore Rd on December 29-30, forcing the evacuation of up to 100 homes.