Australia's fires have now killed at least 29 people, torched more than 15.6 million acres, and destroyed more than 1,400 homes since September.

Before and after: The results of Australia's devastating bushfires

The death toll from NSW’s unprecedented bushfires has climbed to 21 following the death in hospital of an elderly man burned in Cobargo on New Year’s Eve.

The 84-year-old was taken from his home on Tuesday December 31 to South East Regional Hospital in Bega before he was transferred to Concord Hospital in Sydney where he died in the early hours of Saturday.

“A report will be prepared for the coroner,” a NSW Police spokeswoman told AAP on Sunday.

The 21 deaths have occurred across NSW since October as a result of the devastating bushfires that have burned through more than five million hectares and destroyed more than 2100 homes.

The death comes as floods, rather than bushfires, are the new danger in New South Wales.

Desperately needed rain is expected to continue lashing parts of drought-stricken NSW across Sunday and Monday. However, there are fears that the downpour may be a “double edged sword” in some areas.

The state’s northeast and Northern Rivers regions are tipped to receive further showers on Sunday, as a system that has caused bucketing rain and flash flooding in Queensland moves south.

While downpours over the last several days have provided relief for a state ravaged by bushfires – with the number of blazes in NSW dropping from 111 to 73 in six days – experts are concerned.

The Bureau of Meteorology warns that heavy rain could lead to flash flooding and landslips in parts of southeast NSW and the south coast, as the weather system moves towards Victoria over the next 24 hours.

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Most of the eastern & north eastern parts of #NSW received rainfall. Some notable 24 hour totals to 9am this morning were at Clothiers Creek: 250mm, Tumbulgum: 243mm & Bilambil Heights: 212mm. For more details on rainfall received in your area visit: https://t.co/IAC6vtRncr pic.twitter.com/fgG5bRimMy — Bureau of Meteorology, New South Wales (@BOM_NSW) January 18, 2020

Bureau of Meteorology forecaster Abrar Shabren said on Saturday afternoon the effects of the downpour would be felt for at least another day in the northeast and Northern Rivers areas.

“The rainfall will continue for the next 24 hours and substantial rain is expected at those locations,” he said.

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

The Tweed and Northern Rivers regions have so far copped the state’s biggest falls, with Clothiers Creek receiving 250mm in the 24 hours to 9am on Saturday.

On the mid north coast, Grafton had 155mm of rain and Killabakh had 112mm.

While the rain is welcome in fire-affected parts of the state, there are also concerns about the erosion that comes with it.

Despite hopes that rainfall will fill dwindling catchments, some councils fear mass erosion will affect water supplies.

“The soil is kept together by tree roots and other biomass – if enough of that has been burnt away, the catchment will be prone to erosion and even mudslides,” says University of NSW water expert Stuart Khan told the Northern Daily Leader.

“If it rains heavily and too much soil and mud washes into the reservoir, the sediment can overwhelm the treatment process.”

Tamworth Regional Council has reportedly reached out to the NSW Soil Conservation Service for advice – especially in terms of treating water affected by ash – and other councils have done the same.