Sydneysiders have been told to brace for flash flooding this weekend as the city's major dams are set to spill over when a predicted east coast low hits.

The Bureau of Meteorology predicts only moderate rainfall could overfill the Warragamba Dam, which supplies water to more than 3.7 million people living in Sydney and the lower Blue Mountains.

'Warragamba Dam is at 98 per cent at the moment... 30 to 50 millimetres will cause that dam to spill,' NSW SES Deputy Commissioner Mark Morrow told 9News.

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Sydneysiders have been told to brace for flash flooding this weekend as the city's major dams are set to spill over when a predicted east coast low hits. Above are huge waves at Bronte Beach

Sydney is expected to get 5-10mm on Saturday and 20-40mm on Sunday, Weatherzone was predicting on Friday.

Queensland is also risk of flash flooding with a major downpour expected in Brisbane and other parts of the state.

The Bureau of Meteorology has also issued a flood watch for Toowoomba, in Queensland's south-east Darling Downs.

Up to 100mm is predicted for the region over the weekend as the low pressure system moves towards the coast.

This upcoming weather event has sparked the NSW Government to announce $690 million to raise the dam's wall by 14 metres to offer people downstream 'extra protection', Premier Mike Baird told The Sydney Morning Herald.

Up to 134,000 people would need to evacuated if major flooding was to happen.

Above is a weather map showing the amount of rainfall predicted from Friday through to Monday

The Bureau of Meteorology predicts only moderate rainfall could overfill the Warragamba Dam. Pictured is the dam spilling over in March 2012

The east coast low almost two weeks ago added an extra 12.1 per cent of water to Sydney's major dams.

In the week leading up to June 9, Warragamba Dam had 170mm of rainfall and it now sits at 97.7 per cent, according to WaterNSW.

The Upper Nepean, Woronora, Shoalhaven and Blue Mountains got 270mm, 310mm, 280mm and 170mm in the same period.

The areas that will bear the brunt of the weather event will be the region between the Central Coast and Jervis Bay.

Mr Morrow said Sydney's metropolitan areas was also 'prone to flash flooding'.

NATIONAL SEVEN-DAY FORECAST Sydney Melbourne Brisbane Canberra Adelaide Perth Hobart Darwin Friday 21C, clearing shower, 1-5mm rainfall 15C, showers, 1-5mm rainfall 24C, possible shower, 1-5mm rainfall 14C, rain, 1-5mm rainfall 16C, showers easing, 1-5mm rainfall 19C, mostly sunny, <1mm rainfall 12C, showers, 1-5mm rainfall 33C, mostly sunny, <1mm rainfall Saturday 19C, rain, 5-10mm rainfall 13C, clearing shower, <1mm rainfall 25C, possible shower, 1-5mm rainfall 13C, showers, 1-5mm rainfall 15C, possible shower, <1mm rainfall 19C, sunny, <1mm rainfall 13C, mostly sunny, <1mm rainfall 33C, sunny, <1mm rainfall Sunday 19C, rain, 20-40mm rainfall 13C, cloudy, 1-5mm rainfall 24C, possible thunderstorm, 20-40mm rainfall 14C, rain, 20-40mm rainfall 16C, cloudy, <1mm rainfall 20C, late shower, 5-10mm rainfall 13C, mostly sunny, <1mm rainfall 33C, mostly sunny, <1mm rainfall Monday 19C, rain, 1-5mm rainfall 14C, possible shower, 1-5mm rainfall 22C, clearing shower, <1mm rainfall 14C, rain, 1-5mm rainfall 15C, late shower, 1-5mm rainfall 18C, clearing shower, 1-5mm rainfall 13C, possible shower, <1mm rainfall 33C, mostly sunny, <1mm rainfall Tuesday 18C, cloudy, <1mm rainfall 14C, possible shower, 1-5mm rainfall 21C, sunny, <1mm rainfall 11C, possible shower, 1-5mm rainfall 17C, possible shower, 1-5mm rainfall 16C, late shower, 1-5mm rainfall 13C, possible shower, <1mm rainfall 33C, mostly sunny, <1mm rainfall Wednesday 19C, mostly sunny, <1mm rainfall 15C, possible shower, 1-5mm rainfall 23C, mostly sunny, <1mm rainfall 13C, possible shower, <1mm rainfall 16C, possible shower, 10-20mm rainfall 17C, possible shower, 1-5mm rainfall 12C, possible shower, <1mm rainfall 33C, sunny, <1mm rainfall Thursday 18C, mostly sunny, <11mm rainfall 15C, possible shower, 5-10mm rainfall 22C, mostly sunny, <1mm rainfall 12C, possible shower, 5-10mm rainfall 16C, possible shower, 10-20mm rainfall 16C, mostly sunny, <1mm rainfall 12C, possible shower, 1-5mm rainfall 34C, mostly sunny, <1mm rainfall

Vehicles drive through a partially flooded Paramatta Road in Sydney earlier this month when an east coast low hit

Sydney's northern beaches suburb of Collaroy had chunks of its coastline swallowed by massive waves and Tasmania experienced widespread flooding in its northern regions

Brisbane is forecast to get up to 45mm of rainfall across the weekend.

As the weekend's weather event looms, emergency services authorities have warned people to be prepared.

'We will be watching closely and it is a good trigger there for people to understand what the risk is in their area,' Mr Morrow said.

'If you live near creeks that flash flood quickly, or you realise that you are in the middle of heavy rain, then certainly be aware of your surroundings and checkout side and watch the warnings on our systems.'

This east coast low comes almost two weeks after NSW, Victoria, Queensland and Tasmania was lashed by torrential rain and strong winds.