The Department of Conservation believes a fire which ripped through 150ha of land at Ngawha in Northland, threatening a power station, was deliberately lit.
The blaze started on Wednesday night with fire crews continuing to dampen down hot-spots which have flared-up.
The fire also damaged fibre optic cables at the Top Energy geothermal power station forcing its closure for a few days.
Department of Conservation spokesperson Rolein Elliot said she and fire safety officers have found evidence to suggest the blaze was deliberately lit.
Ms Elliot said it appears a pile of pine-slash was set alight, with flames quickly spreading to a nearby reserve.
The fire which swept through bush and pine forest flared up again on Saturday.
The blaze began on Wednesday and was contained on Thursday, but reignited on Friday before being contained again that night.
Ms Elliot said strong winds and warm temperatures caused some concern on Saturday but the situation was under control.
Top Energy chief executive Roger de Brey said staff were inspecting overhead lines, control systems and pipelines at the geothermal power station. He said the cost of loss of generation will be significant.
DoC said fire crews were not deployed as quickly as they should have been to stop the blaze jumping a fire-break.
Ms Elliot said crews were focused on closing the fire-break perimeter on the south east corner, and the change of wind direction took them off-guard.