Bureau of Meteorology NSW forecaster Abrar Shabren said rain is expected across most of NSW from Thursday, barring the state's west and southwest, and could continue until Monday.

The NSW Blue Mountains suburb of Faulconbridge recorded the state's highest rainfall in the past 24 hours with 45 millimetres.

Bushfires burning near the Biriwal Bulga and Cottan-Bimbang national parks in the Mid North Coast region received some help from the rain, with 38.2mm recorded in nearby Mount Seaview.

Areas around the South Coast and Southern Tablelands are predicted to receive up to 30mm but the BOM says the rain will be patchy.

"An inland trough has been deepening across the state and is driving humid air across NSW, leading to thunderstorm activity across most districts," Mr Shabren told AAP on Thursday.

"We will continue to see severe thunderstorms throughout the week, with very heavy, short, sharp bursts of rainfall."

The NSW SES says this could increase the risk of flash flooding, falling trees and landslips where fire has wiped out trees and growth.

"While the rain is welcomed, heavy rainfall and storms in fire affected areas can lead to dangerous conditions such as a higher risk of flash flooding, falling trees and landslips," NSW SES assistant commissioner Paul Bailey said.

"In areas impacted by fires where vegetation has been destroyed, water from heavy rainfall can flow into riverbeds and we could see run-off in areas we wouldn't normally, resulting in flash flooding.

"The NSW SES is also asking residents in fire affected areas to watch for possible landslips as the ground and roads can be damaged, therefore creating a higher risk of a potential slip."

The SES warned residents to prepare their properties by trimming overhanging branches, cleaning gutters and pipes, securing loose items in their backyards and not parking under trees or powerlines.

Conservation authorities, meanwhile, say they have saved the last of the wild Wollemi pine population in the Wollemi National Park from the Gospers Mountain megablaze which is now under control.

Specialist firefighters were dropped into the secret site from helicopters to lay an irrigation system, while fire retardant was also used. NSW Environment Minister Matt Kean said two trees were lost but the species would survive.

"The fire did go through there, we had a few days of thick smoke so couldn't tell if they'd been damaged ... we waited with bated breath," Mr Kean told ABC Radio.

Bureau forecaster Jonathan How says while this week's rain is welcome the end of January and February are expected to be dry.

"Unfortunately the short-term to medium-term outlook is looking quite dry and warm," he said.

Australian Associated Press