Sydney and Brisbane residents will benefit from warm temperatures on Monday before things turn wetter and cooler this week

Thousands of people flocked to beaches, rivers and other waterways across eastern states to cool off, while others enjoyed family outings on Monday.

The Harbor City reached a maximum temperature of 29C with light winds, while in Penrith it reached 31C.

It was slightly warmer in Brisbane at 32°C, 28°C on the Gold Coast and 31°C in Cairns.

Sky News Australia meteorologist Rob Sharp described the heat as “much warmer than usual”, but that will change within the next 24 hours.

Parts of southeast Queensland north of Brisbane and the Sunshine Coast have been warned of a risk of thunderstorms on Monday afternoon and overnight.

The change in weather is expected to continue into Tuesday, with a cold front moving north across Victoria and Tasmania, bringing cool and wet conditions.

Sharpe warned residents of New South Wales and Queensland that a southern change could see Sydney's temperature drop by 10C in just one day.

“We had a stormy change on Friday, the two rainy changes on Saturday and Sunday, and the last of those comes today,” he said.

“But this one looks colder than the others and will impact not just Tasmania, Victoria and southern South Australia, but other states too.”

“A few millimeters” of rain is expected in Sydney from Tuesday morning.

Showers and thunderstorms are also expected across the Sunshine State on Tuesday, with much of the activity concentrated in Brisbane and southeastern areas.

Mr Sharpe said the chance of a shower was about 60 per cent in the Queensland capital and neighboring Gold Coast, but Wednesday was the day of most concern.

The unstable atmosphere is expected to cause strong thunderstorms, damaging winds, hail and flash flooding in parts of the state and across the border into northern New South Wales.

Large parts of Queensland and New South Wales can expect 20mm of rain, while the Sunshine Coast and Bundaberg are expected to see up to 50mm of rain over the next four days.

The weather is expected to persist on Thursday before easing on Friday.