Moisture levels of live trees and shrubs around Sydney are lower than during the Black Christmas fires of 2001 as the region braces for "catastrophic" fire dangers on Tuesday.

Satellite maps to the end of October prepared by Rachael Nolan, a fire researcher at the Hawkesbury Institute for the Environment, show conditions are much drier than normal for woodlands close to the city.

A bushfire burning behind homes near Woodford in the Blue Mountains on Friday. Credit:Wolter Peeters

The moisture content of live plants, particularly leaves and trees, had been falling fairly steadily since March, passing a critical threshold in August. Only the same period in 2002 was drier.

"We already knew it was going to be bad" even before Tuesday's declaration of "catastrophic" fire dangers for the Sydney and Newcastle regions, Dr Nolan said.