Doc staff discovered a "large fire pit" on Te Peka Karara/Stevensons Island in Lake Wanaka this week - a habitat which is home to a breeding programme for protected buff weka.
Doc Wanaka area spokesman Paul Hondelink said the fire was a major concern to staff.
People needed to be reminded about the dangers lit fires posed to natural areas - particularly the islands in Lakes Wanaka and Wakatipu which are home to the buff weka breeding programmes, he said.
"Uncontrolled fires could devastate these natural areas and cause a severe setback to the buff weka programmes which have been developed over nine years," Mr Hondelink said.
A person could be held liable for the costs of fighting a fire if they were shown to be responsible for causing it, said Mr Hondelink.
The recently discovered fire at Stevensons Island was surrounded by beer bottles.
A sign prohibiting fires had been damaged.
No fires were allowed on the lake islands "at any time" of the year, Mr Hondelink said.
The act of vandalism was unnecessary and the clean-up and repair had taken up valuable staff time, he said.
He urged people spending time in public conservation areas to take every care to protect them from fire and to report any incidents at their local Doc office.
A year-round restricted fire season applies to all conservation lands and areas within one kilometre of these lands.
Fires could be lit in the open in these areas with a permit from Doc, he said.
A man was held liable by the Queenstown Lakes District Council for costs associated with fighting a fire on Ruby Island, in Lake Wanaka, in November 2008.
A discarded cigarette caused the fire, which destroyed an area of native vegetation.
