Firefighters battling wildfires in Australia’s most populous state attempted to make headway on Thursday amid favourable conditions, before an "extreme heatwave" hits embattled areas on the weekend.

Thousands of firefighters in New South Wales took advantage of cooler weather and continued to strengthen containment lines.

However, more than 70 fires were still burning across the state, with areas on the south coast currently at the "watch and act" level issued by fire services.

About 12.35 million acres of land have burned nationwide over the past few months, with nine people killed and more than 950 homes destroyed.

A wildfire behind an emergency vehicle near property on Hat Hill Road in Blackheath, New South Wales. Credit: AP

New South Wales has borne the brunt of the damage, with around 850 homes razed in the state.

Authorities are bracing for conditions to deteriorate as high temperatures return.

Sydney is forecast to hit 31C on Sunday before reaching 35C on Tuesday - and the city’s western suburbs could also reach 41C on Sunday.

Bureau of Meteorology forecaster Rose Barr said a heatwave was currently building in southern parts of New South Wales before worsening at the weekend.

"Some areas are forecast to reach extreme heatwave conditions," she said.

"With the increasing heat and winds, the fire danger will worsen into the new week, with Monday and Tuesday most likely to be the most significant fire weather days."

A house on fire from a bushfire in Gumeracha, South Australia.

Fire danger ratings remained very high in north-western New South Wales, and high in Sydney.

Meanwhile, South Australian firefighters were battling wildfires in Adelaide Hills, which has been downgraded to the "advice" level.

South Australia state last week had 86 homes destroyed after wildfires flared in catastrophic conditions.

A return of extreme temperatures, however, are expected with Adelaide, the state capital, set to reach a sizzling 40C on Friday at the start of a four-day heatwave - its second such hot spell in just over a week.