Urgent warning as Warragamba Dam threatens to spill over during record-breaking deluge

Emergency services have warned a major dam in Sydney's outer south-west will likely begin spilling on Saturday evening, as the state is lashed with heavy rain.

At about 1.30pm, Water NSW said Warragamba Dam - which has a capacity of 2,065 gigalitres - was 'close to full capacity', and would likely spill over within hours.

Most recent modelling suggests the dam could reach a spill rate of about 40GL per day, if the catch receives the forecast amount of 35mm of rain over the weekend.

WaterNSW said this would be less than the spill peak from April, which exceeded 200GL a day.

A number of other dams across Greater Sydney are also likely to spill, including Cataract, Nepean, Woronora, Tallowa, Greaves Creek, Medlow Bath, and Middle Cascades.

The Bureau of Meteorology has also warned of the potential for severe thunderstorms across several areas of NSW, including parts of Sydney, the Hunter and the Illawarra, with the potential of localised flash flooding from the heavy rainfall.

Parts of the NSW south coast have been issued with severe weather warnings as 70 to 200mm of rain could fall.

Surf conditions will also be severe, the bureau forecasts. Hazardous surf warnings are in place for the Macquarie, Hunter, Sydney and Illawarra coasts Saturday, and Hunter and Sydney for Sunday.

Parts of the state's south, including the Illawarra, the South Coast, Southern Tablelands and Snowy Mountains have been issued severe weather warnings, with heavy rain and flash flooding forecast.

Rainfall of up to 90mm is expected over just six hours on Saturday in southern parts of the state, with possible isolated rainfall of up to 120mm in some areas.

Earlier, a bureau meteorologist said there was potential for severe thunderstorms about the Sydney metropolitan area and Illawarra, to bring heavy rainfall that may lead to flash flooding.

Anywhere from 20 to 45mm of rain is expected around Sydney on Saturday, with three to 20mm on the outlook for Sunday.

There was the potential for up to 200mm of rain within 24 hours in some southern parts of the state and a flood watch would be issued, the meteorologist said.

The bureau has issued warnings that heavy rainfall could lead to flash flooding in northern parts of the South Coast, southern parts of the Illawarra, eastern parts of the Southern Tablelands and far northeastern parts of Snowy Mountains districts from Saturday morning into Sunday.

'Six-hourly rainfall totals between 70 and 90mm, with isolated 100 to 120mm possible. 24 hourly totals between 80 to 120mm are likely and isolated falls up to 200mm are possible over the ranges,' the forecast reads.

NSW Police is urging residents to 'consider staying out of the water and avoid walking near surf-exposed areas'. 

Sydney has smashed its record rainfall for the start of May with nearly 150mm recorded in the Observatory Hill gauge since the start of the month.

The bureau reckons up to four millimetres could fall on Monday with a 20 per cent chance of any rain Tuesday.

If the rain clouds stay away that will snap Sydney's 13-day wet streak.

OTHER STATES

Meanwhile, the weekend's storm is expected to also bring rain to other parts of the country, with Queensland expected to face rain and thunderstorms.

Queensland's south-east is set to face most of the state's deluge, with Brisbane set to remain wet for both Saturday and Sunday.

Canberra, as well as northern and eastern parts of Victoria, will also be facing an entire weekend of rain.

Over in WA, thunderstorms were predicted on Friday evening for the Margaret River and Busselton areas.

But residents were shocked at 4pm local time when a tornado tore through the town of Bunbury, causing widespread damage and thousands of homes to lose power.

A PCYC building with dozens of young people inside was completely destroyed, and many people injure