A bird ignited by powerlines caused Tarago fire, investigators find

"The fire started as a result of a bird flying close to high-voltage powerlines, igniting and landing in dry grass on the 17 January 2017," the NSW RFS fire investigation report read.

NSW RFS deputy incident manager Andrew Gray said investigators found the bird carcass in the scorched fire zone directly under the powerlines west of Bungendore Road and near to wind turbine towers.

Mr Gray said the source of the fire was "a fluke" but high temperatures, dry ground fuel and strong winds made for dangerous conditions.

"All we needed was a spark and our bird going through the powerlines causing them to arc and ignite his feathers has dropped to the ground alight and into the grass."

While some media organisations had reported it was a crow Mr Gray said it was difficult to determine the species from the charred carcass.

"He has certainly come off second best and was in a very blackened condition," he said.

Despite wet weather across the region on Friday morning, 40 firefighters from Eurobodalla were on the fire ground deepening containment lines and giving Lake George crews some much-needed rest.

The blaze, which tore through almost 3400 hectares north of Canberra, is now under control, but still burning in the area near Taylors Road, Bungendore Road and Mount Fairy Road.

Building impact analysis teams were assessing damage across the substantial Tarago fire area, while fire investigators and ACT Policing worked to determine the source of a grass fire that began at Mulligans Flat near Sutton on Wednesday.

Mr Gray said it was unlikely the Tarago fire would be extinguished today and crews were patrolling the area to ensure the fire remained within its current footprint.

Fire danger ratings for NSW southern ranges were still set to high and it was likely fire could burn in dry root systems of eucalypts, tea tree and pines for days to come.

"We will be requesting Lake George Crews to assist us on Saturday and Sunday to patrol the fire," he said. "We'll continue until this thing is completely out."