Widespread rain is forecast for much of New South Wales with warnings in place for severe weather and possible flooding in some parts of the state.

An initial flood watch is in place for possible riverine flooding along the coastal rivers from the Tweed to the Hastings from Thursday.

A surface trough over central NSW and Queensland was beginning to bring rainfall in areas from Goondawindi to Coonamble on Wednesday.

Heavier falls are expected along the north and north-east coast of NSW on Thursday and Friday before the rain extends southwards into Sydney and the South Coast over the weekend.

The Bureau of Meteorology said it was possible the rainfall would extend to parts of the state with active bushfires as the weather system moves south.

But Grace Legge, a senior meteorologist at the Bureau of Meteorology, said it was too early to forecast how much rain would make it to those areas.

She said while rainfall was welcome, fire grounds were also more vulnerable to severe weather that could increase runoff of ash and silt into waterways.

WaterNSW is monitoring forecasts for areas around the Warragamba Dam, the source of 80% of untreated supply for greater Sydney.

The extent of any water quality impacts on water in the dam will depend on the timing and intensity of the rainfall.

Two silt curtains designed to trap material that could pose a risk to water quality have been in place since January.

“WaterNSW is working closely with the BoM to anticipate potential rain impacts in coming days,” a spokesman said.

At this stage, the BoM is forecasting widespread rain of 20mm to 50mm for central Queensland and eastern NSW, with the potential for up to 100mm in some areas.

The NSW Department of Primary Industries has warned agricultural producers to prepare their properties and stock for potential effects from heavy rain, thunderstorms and flash flooding.

The department said on Wednesday the heavy falls would also increase the risk of organic debris ending up in waterways, which could reduce water quality and “may trigger fish deaths in already stressed river systems”.

Several fish kills have already occurred this year as the result of ash from bushfires, and other organic matter and sediment, washing into local catchments.

“Despite these short-term risks, the resumption of flow and subsequent replenishment of refuge holes and connectivity along waterways will provide long-term benefits to native fish populations during the current drought period,” a spokeswoman said.

In Western Australia, a tropical low off the north coast is expected to develop into a tropical cyclone on Thursday morning.

Legge said it was expected to deepen and could become a category three cyclone by Saturday as it moved toward land.

She said there was still “large uncertainty” as to whether it would make landfall, but if it did that could occur in the region from Port Hedland down to Onslow on either Saturday or Sunday.

On Wednesday there was also a flood watch in place for parts of the western and northern Kimberley.