A month's rain has fallen in just a few hours in some areas while "giant" gold ball sized hailstones have rocked towns as a 'perfect super cell storm' makes its way across New South Wales and southeast Queensland.
The Bureau of Meteorology (BoM) has issued a severe thunderstorm warning as heavy rain, large hail stones and wind gusts of up to 125 kilometres per hour lash parts of northern and central NSW.
At 6pm the Bureau reiterated it's warning that dangerous conditions could eventuate, only an hour after saying the threat of storms had "temporarily eased".
The weather itself didn't heed the temporary reprieve with a severe storm hitting west of Penrith, in Sydney's west, about 5pm. That storm then headed over the Blue Mountains with heavy rain and hailstones pummelling Katoomba and Blackheath. Moore, in the state's north could see "giant hail", the bureau warned. While the Central West city of Dubbo has been battered by hailstones.
However, Sydney, Wollongong and the Central Coast appeared to have been spared the worst of the weather with severe thunderstorms no longer expected - at least for Monday night. Southern Queensland could also be in the firing line.
In the Central Coast, electricity provider Ausgrid said 3000 homes were without power.
The BoM has predicted the worst-hit locations will be in the central and northern areas of the state. A severe warning has been issued for Tenterfield, Dubbo, Tabulam, Drake, Baryulgil and Narromine.
Dubbo is already feeling the brunt of the storm, with heavy rain, large hailstones, and flash flooding already occurring in some areas.
The Central West city has exceeded it's entire average March rainfall since 9am Monday.
NSW SES spokesman Phil Campbell says emergency services are "quite concerned" about the supercell thunderstorms.
"We're asking people up in that northeast part of the state and also inland around Tamworth and Moree just to make sure they're well prepared," he told AAP on Monday.
"At the moment, we've not had any calls for assistance, which is good news. "We do have a number of weather models, according to the bureau, that are forecasting very heavy rain from the middle to late part of the week. We're just keeping an eye on that."
Many streets within the town have been converted to shallow canals, with water lapping the gutters, and residents reporting hailstones the size of golf balls, particularly in the city's east, the ABC reported.
Extra emergency crews have been called into Dubbo and the State Emergency Service (SES) says it has so far received 25 call-outs for help.
Other location that might be affected include Armidale, Orange, Mudgee, Gulgong, Tamworth, Gunnedah, Moree, Narrabri, Walgett, Dubbo, Parkes and Lightning Ridge, according to the bureau.
And if you're living in New South Wales and Queensland you can also expect heavy rain, huge hail and flash flooding as "perfect storm" conditions are set to batter both states this week.
A low pressure trough has started to move east into New South Wales this afternoon, and looks set to collide with a second trough, causing heavy rainfall and hail.
The systems are combining to create the "perfect super cell storm" conditions which are set to unleash across large parts of Australia's east coast and further inland.
Meteorologists said there was also a chance the super cells could spawn tornadoes, however they stressed the possibility of this occurring remained low.
Sky News Weather metrologist Tom Saunders said it was possible tornadoes could form in areas such as inland NSW with strong winds supporting them developing.
"It remains in the realms of possibility," he said.
"These super cell storms are the most dangerous type. "
Sky News Weather meteorologist Tristan Meyers said Sydney was likely to escape the brunt of the storms - at least for today, but plenty of rain remained on the cards.
He warned the city and the greater Sydney area can expect to cop an absolute drenching tomorrow.
Sydney is already feeling the effects of the wild weather, and according to forecasters things are looking increasingly wet for a while.
Up to 70mm of rain is predicted to fall tonight and tomorrow, the Daily Telegraph reported.
While the severe storm warning has been cancelled for Queensland this morning, Mr Meyers said that didn't mean it couldn't develop later today with Brisbane set to cop heavy falls tomorrow and Wednesday.
"The storm warning for NSW remains in place and has the potential to bring damaging winds, and hail," Mr Meyers said.
"Unfortunately we have the perfect conditions which will bring widespread storms over large areas.
"A low and upper-pressure trough combined with a lot of cold air is creating a lot of instability and perfect conditions for hail."
Mr Meyers said the storm activity will increase in intensity and become more widespread by this afternoon across both states.
Damaging wind gusts over 90km/h and hail 2cm in diameter is also predicted with warnings for people to put cars undercover and secure loose items.
"I hope you enjoyed the nice weather over the weekend because the persistent rains are here and it looks like they're hanging around all week," he said.
New South Wales
A severe thunderstorm warning has been issued for an area stretching from Griffith in the Riverina all the way up to Moree and the Queensland border including Sydney and Wollongong.
Large hailstones, damaging winds and heavy rainfall, which could lead to flash flooding, have been predicted across the Central and North West Slopes and Plains, the Hunter, Riverina and parts of the Central, Lower Western and Northern Tablelands today.
At 5pm on Monday, the BoM said "severe thunderstorms in the warning area have temporarily eased (but) further severe thunderstorms are still possible".
The Bureau of Meteorology initially issued a warning for the northern and central parts of the state early on Monday, but the warning area was expanded before midday, AAP reported.
Numerous areas, including Tenterfield, Dubbo, Tabulam, Drake, Baryulgil and Narromine, are in the path of a low-pressure system that's moving across the state, according to BoM.
NSW SES spokesman Phil Campbell said emergency services are "quite concerned" about the supercell thunderstorms.
Mr Meyers said most of the storm's fury today would stay west of the Great Dividing Range but meteorologists were continuing to keep an eye on Sydney.
The CBD and western suburbs will see showers each day this week, with heavy falls predicted tomorrow likely to lead to flash flooding.
"Even moderate falls will bring a massive impact in Sydney," he said.
He said severe falls would hit Sydney tomorrow with up to 50mm expected across large parts of the basin.
The city can expect cloudy conditions and a top of 27C today and possible storms.
Heavy falls are predicted for tomorrow with a top of 26C and further showers and 26C on Wednesday.
Showers are expected to continue until Sunday.