The Gospers Mountain mega-blaze northwest of Sydney, which has burned over 512,000 hectares over the last two-and-a-half months, was on Sunday finally brought under control.

"Containment took longer than expected due to unfavourable weather conditions, however due to our hardworking crews, we have achieved that today," the Hawkesbury RFS said on Facebook.

"It is important to remember not to be complacent as there are still a few months of the bush fire season to go with some bushland that still has not been burnt."

Rain forecast for most of NSW this week is set to provide relief for firefighters and bushfire-ravaged communities.

The Bureau of Meteorology is expecting significant rainfall for most of the state to arrive mid-week and hang around until the weekend.

Rain is expected on parts of the state most severely affected by bushfires in recent weeks, with the Snowy Mountains and South Coast forecast to receive rain and possible thunderstorms on Wednesday.

Sydney is expected to receive its most significant downpour in several months, with forecasts of 2-8mm on Thursday and 5-10mm on Friday.

BOM meteorologist Gabrielle Woodhouse said while rain would be welcome at the fire grounds, it may also bring dangers to fire-affected landscapes.

"We are looking at a couple of days in a row of some showers and thunderstorms, some of which may produce significant accumulation over those couple of days," she said.

"It will be quite welcome but there are some extra dangers and risks associated with it as the landscape is quite vulnerable with the fire damage.

"We've lost a lot of vegetations and there is the risk of landslips."

Across NSW on Sunday night there were 122 fires burning, but none were at emergency levels.

The Department of Defence said they would use favourable conditions over the next few days to create a 70km-long, 1km-wide firebreak in the Snowy Mountains region.

Prime Minister Scott Morrison on Sunday paid tribute to 19-year-old Courtney Partridge, who died on November 29 last year after suffering an asthma attack as a result of smoke in Glen Innes in the New England region.

"The sheer sense of loss, pain, hurt, grief, frustration, fear, particularly well away from the fires where we've seen also that terrible loss of the young girl as the result of an asthma attack," Mr Morrison said.

"This has I think, created an environment where people for the first time have wanted to see a more direct involvement of the federal government in responding to these national disasters."

RFS deputy commissioner Rob Rogers confirmed on Sunday that 2136 NSW homes have been destroyed this fire season.

More than 1200 of those homes have burned down since New Year's Eve.

The Catholic Diocese of Sydney held a special service on Sunday for bushfire victims and drought-affect communities at St Mary's Cathedral.

Sydney Archbishop Anthony Fisher said many Australian priests had flocked to the south coast to assist, or were serving as army reservists.

Australian Associated Press