The Royal National Park remained closed on Sunday, a day after wildfires forced hundreds of people to be evacuated by boat.

Approximately 200 people, including those attending a junior surf carnival, became stranded in the park as the flames swirled around them. The Rural Fire Service (RFS) used boats to rescue them from beaches and bring them to safety.

Firefighters and water-bombing helicopters slowly managed to bring the blazes under control, but the column of thick black smoke was clearly visible over the southern suburbs of Sydney.

More than 50 blazes were reported around the state, and numerous more were seen in South Australia and Tasmania.

The fires spread rapidly due to the hot dry weather, which has been affecting much of southeastern Australia.

In Melbourne, the temperatures peaked on Friday, with Avalon Airport soaring to 42.8 degrees Celsius and Laverton reaching 42.7C.

The heatwave came to an abrupt end when a cold front came through at 2:00pm local time, allowing the temperatures to readily fall over the following hours.

The temperature in Sydney, however, is still rising and is expected to peak on Monday.

The city itself is forecast to top at 33C, but the western suburbs are expected to experience far higher temperatures.

Penrith and Richmond are both expected to see temperatures soaring to 43C, and a severe fire danger is forecast for the Greater Sydney Areas, as well as the Southern and Central Ranges.

The heat is also stretching the power supply to its limits, as customers turn up their air conditioning. People are being warned to prepare for possible power cuts.