A discarded cigarette butt is believed to have caused a fire on the island last Friday, a preliminary investigation by Queenstown Lakes District Council inspectors has concluded.
QLDC Rural Fire chief Gordon Bailey said investigators had a suspect, believed to be from Wanaka, who was boating on the lake on Friday and had visited Ruby Island shortly before the fire started.
The cause of the fire was "not malicious -more careless" in its origin, he said.
"In the first instance we will be seeking to recover the costs associated with bringing the fire under control."
Costs could run into "several thousands of dollars", he said.
A helicopter was called in to douse the fire with a monsoon bucket, several powerboats were commandeered, and about 20 Wanaka fire brigade members battled for almost two hours to bring the blaze under control.
Rural fire investigators Tim Francis and Nathan Keenan travelled to Ruby Island on Friday evening to map the fire area.
The pair - who work for council consultants Lakes Environmental - spoke to several groups of people who were either on the island, or boating on the lake at 2pm when the fire started, as part of their investigation.
Mr Bailey said fires were often started by discarded cigarettes.
"There is a lot of time, money and resources spent fighting fires, because people don't bother to properly stub out a cigarette," he said.
A fire in 1992 virtually destroyed Ruby Island and a revegetation and restoration programme implemented afterwards was threatened by another blaze in 2005.
