Cold snap overnight sends Sydney's temperature plummeting 10 degrees overnight to just 21C – with rain set to drench Australia's east coast

Sydney's temperature has plummeted more than 10 degrees overnight after residents made the most of the October long weekend by hitting the beach.

The Harbour City reached 33.7C in the CBD and 34.3C in the western suburb of Penrith by 2.30pm on Monday.

But the sweltering, above average weather came to an abrupt end late on Monday night as a gusty southerly change blew across the city. 

The unseasonably warm conditions also dropped in Melbourne after groups gathered in mass at St Kilda Beach. 

Crowds flock to Bondi Beach in Sydney's eastern suburbs on Monday as the city enjoys its hottest day since March

The harbour city reached 33.7C in the CBD and 34.3C in the western suburb of Penrith by 2.30pm on Monday. Pictured: Beachgoers at Bondi Beach 

Sydney is expected to reach a maximum of 21C on Tuesday, with light showers forecast. 

The Bureau of Meteorology said the conditions will be cloudy, with south to southeasterly winds at 15 to 25km/h.  

Weatherzone meteorologist Craig McIntosh said Monday was Sydney's hottest day since March.

The cool change came through late on Monday night.

'The winds really ramped up from the south after 10pm. It was the strongest around 11pm around last night,' he said. 

'That southerly change is what dramatically brought the changes today and brought with it cloud cover.

Sydney is expected to reach a maximum of 21C on Tuesday, with light showers forecast. Pictured: Sunrise on Tuesday

Electronic signs at the beach warned visitors that the allowed capacity was close to being reached on Monday

'Despite that, the 21C forecast is average for this time of year.' 

Sydney could also be in for a drenching over the coming days. 

'A brief shower this evening, less than a millimetre,' Mr McIntosh said. 

Mr McIntosh said between one and five millimetres could fall in Sydney on Wednesday and Thursday. 

There is also a chance of a thunderstorm from early afternoon on Thursday. 

Brisbane will be partly cloudy throughout the week, with a maximum of 27C forecast for both Tuesday and Wednesday and 28C on Thursday.

Queensland's capital city will end the week with sunshine on Friday, with highs of 31C. 

Locals in face masks sunbathe at St Kilda on Saturday as Melbourne sweltered through unseasonably warm weather. The city suffered a cool change on Sunday night

Melburnians have started the week with rain and a dip in temperature after flocking to the beach over the weekend. 

The city reached 29.5C on Sunday before a cool change hit in the evening. 

The bureau has forecast a maximum of 14C on Tuesday, with a shower or two. 

The rain will continue through the week, with the temperature unlikely to scale past 19C.  

Adelaide will experience a shower or two and the temperature will linger around the high teens throughout the week, while Perth will hover between 20C and 25C. 

Thunderstorms are likely in Darwin with the mercury to hit 30C most days. 

Rainy conditions are forecast for Canberra with highs of 19C.  

Police patrol St Kilda Beach in Melbourne amid fears residents would breach coronavirus restrictions 

Sydneysiders made the of the warm weather over the long weekend, packing out Bondi Beach and other popular spots.

Cronulla beaches were being closed on Monday afternoon after reaching capacity, Surf Life Saving Sydney said.

'Please do not head down to the beach today,' it said in a Facebook post.

Waverley Council earlier advised Bondi Beach was nearing capacity and access to the sand and water would soon be restricted to allow for social distancing.

'We do not like having to restrict access to the beach so as a reminder to everyone if beach access is closed or the sand looks busy, please come back another time or use that time to visit our wonderful cafes and other local businesses,' a spokesperson said.

Locals soaked in the rays at Bondi Beach as New South Wales had its tenth day in a row without any COVID-19 community transmission on Monday

Randwick City Council also said many of its beaches were approaching capacity.

Northerly winds were responsible for Sydney's long weekend hot weather. 

'The final day of the long weekend will be the warmest with temperatures reaching 29C in Sydney and 33C in Blacktown and Penrith, about seven degrees above average,' Weatherzone's Felix Levesque said.

The bureau said: 'Northerly winds ahead of a low pressure trough bringing another warm and mostly sunny day to Sydney, ahead of a gusty southerly change along the coast late this evening, and a cooler, cloudy though mostly dry Tuesday'. 

FOUR DAY WEATHER FORECAST 

 SYDNEY:    

TUES: Max 21. Possible light shower.

WEDS: Min 15, Max 22. Possible late rain.

THURS: Min 16, Max 28. Shower or two.

FRI: Min 13, Max 25. Sunny.

MELBOURNE: 

TUES: Max 14. Shower or two.

WEDS: Min 10, Max 18. Early fog. Rain developing.

THURS: Min 13, Max 19. Rain.

FRI: Min 11, Max 19. Shower or two.

ADELAIDE: 

TUES: Max 18. Partly cloudy.

WEDS: Min 9, Max 16. Showers developing.

THURS: Min 9, Max 17. Shower or two.

FRI: Min 9, Max 21. Mostly sunny.

CANBERRA:    

TUES: Max 19. Rain at times.

WEDS: Min 11, Max 19. Rain.

THURS: Min 12, Max 19. Showers.

FRI: Min 7, Max 17. Partly cloudy.

BRISBANE:

TUES: Max 27. Partly cloudy.

WEDS: Min 15, Max 27. Partly cloudy.

THURS: Min 16, Max 28. Partly cloudy.

FRI: Min 17, Max 31. Sunny.

PERTH: 

TUES: Max 23. Mostly sunny.

WEDS: Min 10, Max 24. Partly cloudy.

THURS: Min 13, Max 21. Shower or two.

FRI: Min 11, Max 22. Partly cloudy.

HOBART: 

TUES: Max 16. Partly cloudy.

WEDS: Min 5, Max 17. Mostly sunny.

THURS: Min 8, Max 16. Rain.

FRI: Min 11, Max 16. Rain.

DARWIN: 

TUES: Max 33. Shower or two. Possible storm.

WEDS: Min 25, Max 32. Shower or two. Storm likely.

THURS: Min 25, Max 32. Shower or two. Storm likely.

FRI: Min 25, Max 34. Shower or two. Storm likely.

SOURCE: Bureau of Meteorology