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Fire crews have battled two scrub fires in the Dublin Bay area near Wanaka in as many weeks with the one on Thursday burning an area of 100m by 500m.
Otago Rural Fire Authority fire investigator Jamie Cowan said because the source of the blaze was only about 1.5m away from Dublin Bay Rd it would be very hard to determine the cause.
It was unlikely to be camping related, he said.
"What can cause a fire in that place and it can only really be a spark from a vehicle or cigarette being thrown out of a window or similar."
Foul play was not being ruled out but was extremely unlikely, Mr Cowan said.
Without more significant information the cause was unlikely to be determined.
He was also a worried people were dumping ashes in the area and could cause another major blaze in the area.
A pile of ashes from what appeared to be a barbecue were found near the fire on Thursday.
The ash piles were not related to either fire.
It appeared the ashes were dumped under trees and bushes without being watered down, which a major concern for fire authorities, Mr Cowan said.
"There are a huge number of fires, both house and vegetation fires, each year caused by ash being dumped ... In both these cases the ashes were dumped without being wet down, which is an absolute sitter for causing a fire."
He was unsure exactly what sort of device the ash had come from but was concerned there could be a new type of barbecue being used by campers, Mr Cowan said.
"I saw one in a camping area I was in a couple of weeks ago and I didn’t think too much of it. Then I saw another couple yesterday near the site of the fire.‘‘It may well be a small portable barbecue or it may be a cool new device campers are using that I don’t know about."
If it was a new type of cooking device, he wanted to know more about it so he could run a risk assessment on it, Mr Cowan said.
An investigation into the February 24 Dublin Bay fire was almost complete but it would be another month or more before it was made public.
"I have a few more leads to follow up with the police, but I should be finished by early next week."
Once he had finished the investigation, his findings would be passed on to the New Zealand Rural Fire Authority, he said.