A "substantial ran event" is forecast to be heading for large parts of eastern Australia, which has been struggling with an extreme drought.

A weather system is expected to travel across Australia from the west over the coming days, local website News reported.

When it hits South Australia it will take a northeast turn and barrel straight for New South Wales (NSW) and Queensland. Parts of Victoria and Tasmania could also feel the effects.

Around 50mm of rain could fall in Sydney towards the end of the week and into the weekend according to the Bureau of Meteorology (BOM); 40mm in Brisbane and as much as 80mm in northern NSW and 65mm into the Gold Coast.

New South Wales, Australia's most populous state, which produces about a quarter of the country's agricultural output, was officially declared entirely in drought on August 8.

For the entire month of August so far Sydney has only received 1.4mm of rain and Brisbane only a touch more at 3mm. The average rainfall for this time of year in Sydney is around 55mm, so if the upper end of the BOM forecast is realised that could see much of the month’s moisture pour down in just days.