Sydney’s Warragamba dam expected to spill as parts of city’s west prepare for flooding

Parts of Sydney’s north and west could face flooding with the latest big rain event predicted to cause the city’s main dam to spill.

WaterNSW is forecasting Warragamba dam will reach full capacity and begin overflowing into the Hawkesbury-Nepean River downstream from Friday evening.

At the upper end of predictions, the dam’s catchment area will receive as much as 100mm of rain from the current event. If the forecasts are correct, Warragamba’s spill could reach a peak rate of as much as 100 gigalitres a day by Saturday morning, WaterNSW’s modelling shows.

Such a flow, though, would only be one-fifth of the rate during March’s extensive flooding in the region, a WaterNSW spokesperson said.

Related: Torrential rain to hit NSW and Queensland this weekend amid wet summer outlook

“Downstream impacts are likely and will be determined by the spill volume combined with downstream tributary flows,” the official said.

Waragamba was at about 99% full on Thursday with the city’s overall reservoir network sitting at 95.5% full.

On Friday morning, the Bureau of Meteorology issued a weather alert, advising motorist to “take extreme care”.

“Flooded roads and reduced visibility in heavy rain will make driving conditions dangerous during Friday in all suburbs,” the warning said.

The bureau earlier this week declared a La Nina event is under way in the Pacific. The weather pattern, combined with other influences, typically means eastern Australia will have above average rainfall for the coming summer. Insurers are among those gearing up increased damage bills from floods and a more active cyclone season than usual.

The bureau’s latest seasonal forecast, issued on Thursday, also pointed to a cooler than average summer for eastern parts of the summer at least for daytime temperatures.

Related: What is the meaning of La Niña and how will the weather event affect Australia’s summer?

Sydney’s forecast was for as much as 45mm of rain on Friday. It received 21mm in the 24 hours to 9am.

Rainfall totals in that period also included almost 50mm for Richmond to Sydney’s north-west and about 43mm for Penrith, to the west.

On Thursday, authorities said they had been increasing releases from Hume Dam into the Murray River gradually over the past week in anticipation of the rain. Thursday’s release was about 30GL.

“There remains a high level of uncertainty around the weather pattern. If rainfall continues to be intense and generate significant runoff in the next few hours, releases will rise further to 35 GL per day, with the potential for further increases over the next day or two,” the Murray Darling Basin Authority said in a statement.

The bureau has an initial flood warning out for minor flooding at Corowa on the Murray.

Many inland rivers, such as the Gwydir, Lachlan, Macquarie, Murrumbidgee and Namoi, face the prospect of flooding, with dams full and widespread rain falling or expected.

Meanwhile, in Queensland, a man’s body was found in a ute submerged in floodwaters.

A passerby noticed the ute and dead man near the Gregory Highway in Hibernia, west of Rockhampton, about 7am on Friday. They alerted police and emergency services who were working to get the ute out of the water.

“Recovery of the vehicle may take some time due to the location and current weather conditions,” a police spokesperson said in a statement.

The forensic crash unit is probing the man’s death. It is not clear how he died.

Detectives are unsure whether the man’s car was swept off the road and he drowned, or if he died as a result of a prior crash or medical episode and the ute was later washed away.

Heavy rains have been lashing the region for two days with falls up to 270mm in some places and localised flash flooding.