The fire is likely to increase as crews continue their backburning operations, with parts of the fire in inaccessible country.

Department of Sustainability and Environment incident controller Jon Rofe said Raglan residents needed to remain alert, although fire behaviour had decreased this morning.

''The main part of the fire is 6km north west of Raglan and the fire is gently backing down the south western slopes of the mountain towards the private property in those areas,'' he said.

''No houses have been lost but there will have been some private fences damanged and we will have people assessing that.''

As well as the 120 fire crews, nine aircraft including six waterbombing aircraft and three observation aircraft, are helping efforts.

With the current north-west winds, Mr Rofe said the main area of concern was to Raglan and surrounds.

Raglan residents were warned to prepare to leave their homes this morning with the fire expected to impact between 8.30am-10.30am.

The fire is part of the Buangor Ferntree Waterfalls fire in the Mount Buangor State Park.

It is listed as medium and slow-moving, is travelling south and is creating spot fires up to 500 metres ahead.

The Country Fire Authority has urged residents to activate their bushfire survival plan and said for those away from their homes, do not try and return as the roads could be dangerous.

Tourists are advised to stay away from the area.