Interstate help for NSW fire crisis

Metropolitan firefighters from interstate will back up hundreds of NSW colleagues who are heading out to help rural crews in the bushfire emergency.

Up to 170 NSW Fire and Rescue tankers and engines with almost 800 officers are being sent to fire-ravaged communities in the Blue Mountains on Tuesday.

Ten crews from Queensland and 10 from Melbourne will arrive in Sydney on Wednesday to man metropolitan posts.

It's one of the largest mobilisations in the history of Fire and Rescue NSW, Commissioner Greg Mullins said.

He said the fire threat in the Blue Mountains was unparalleled and warranted the "unprecedented number" of resources.

Firefighters are battling to get on top of the Blue Mountains fires ahead of worsening conditions on Wednesday.

The 1994 bushfires and 1997 Sydney hailstorm are the only other incidents that required such widespread mobilisation.