Police now believe a fire at Paritutu Rock, near New Plymouth, about 9.30pm last night is suspicious.
Witnesses told police that a group of people on the rock were setting off fireworks about the time the blaze started.
Detective John Simes of New Plymouth CIB said he would like to speak with anyone from that group or anyone that may have information that would assist to identify members of that group.
Fire communications told the Herald last night that as crews arrived to extinguish the fire, it began to rain. By about 11.30pm, it started to rain heavily which put out the blaze.
Police warned that temperatures were still high and vegetation is very dry and urged people to "please take extra care" when using anything that may start a fire in outside areas.
Any information can be given to Detective Simes on 06 215 4178 or crime stoppers on 0800 555 111