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.