It's been a long night for firefighters in New South Wales, where three bushfires merged to create a massive blaze in the Snowy Mountains.

The 233000-hectare Green Valley fire merged with the nearby East Ournie Creek blaze and the huge 312000-hectare Dunns Road fire on Friday evening.

NSW Premier Gladys Berejiklian said the extent of the damage wouldn't be known until later this morning.

The "mega-blaze" straddles the New South Wales and Victoria borders and covers more than 640,000 hectares.

Firefighters warned a southerly change was moving rapidly across NSW and had caused some fires to spread quickly.

That change was not expected to reach some coastal areas until early this morning.

The NSW Rural Fire Service said the predicted southerly wind change did not eventuate across the Blue Mountains fire zone, which helped to ease fire activity.

The RFS added there were 147 bush and grass fires burning across NSW – 65 uncontained – as of 6am AEDT today.

Conditions have eased across most fire grounds, with only 1 fire at watch and act warning in place.

RFS crews will make the most of more favourable conditions to work on containing these fires.

In Victoria, strong winds of up to 100kph sparked ember attacks on the communities of Swifts Creek and Omeo overnight.

Premier Daniel Andrews warned it was a "challenging time".

A bushfire emergency warning was in place for Davenport Village, Dinner Plain, Flourbag and Hotham Heights.

A watch and act warning was in place for a large area stretching east from Mallacoota, north past Burrowa-Pine Mountain and west past Dandongadale. The bushfire zone reaches as far as Orbost in the south.