Rain across firegrounds and drought-hit plains from Thursday

That's Monday's tweet from the NSW Rural Fire Service facing its biggest fire threat in modern history. It's also the sentiments from farmers facing their worst drought in recorded history.

A solid run of rain starting from Thursday is expected in many areas of the state, stretching from the Victorian to Queensland border.

At the moment, the Bureau of Meteorology's computer generated forecast rainfall model (above) has upwards of 50mm falling in some of the worst drought-affected places in the state including on the Liverpool Plains, with good rain possible through the main grain belt.

A forecaster for the BOM said the computer model needed to be assessed along with other weather data, and it may show more rain totals than actually fall. But the model, if it does eventuate, shows impressive totals for rain up until at least January 20.

The BOM said though there was definitely about four to five days of rain on way for much of eastern NSW, with moisture flowing into a trough from the Tasman Sea. The trough will move up and down the state until at least Sunday.

The rain will mainly be in showers and storms, with heavier falls in some areas. "Some may get 10mm, some will get 20mm and others more, it could be quite variable," the BOM forecaster said.

It also means an end to the movement of heatwaves into the state from central Australia for at least a week and temperatures will be milder. The rain event coincides with the climate shaper, the Indian Ocean Dipole (IOD), moving into a neutral phase, allowing more moisture to stream down from north into the continent.

The rain event also follows the start of the monsoon in northern Australia, with already two cyclones off the coast of Western Australia. An island south-west of Darwin recorded the highest ever rainfall total for a day in the Northern Territory (Dum In Mirrie, 562mm in 24hours).

At the moment, totals were changing on the BOM website. The forecast for Dubbo in the Central-West shows the city recording, as a minimum, about 20mm by Sunday and possibly double that (with 70-80pc certainty of this occurring).

The BOM warned though that some storms may bring heavy rain.

The main beneficiaries will be the coastal areas, the forecaster said, but on Sunday the trough will move over central areas bringing more rain.

The north-east of the state will also get more than its fair share of the rain. There has already been some good falls on parts of the Northern Tablelands in the past week including Armidale with 39mm, Gowan Brae with 88mm, Tenterfield with 23mm and Deepwater with 58mm.