Australian Prime Minister Scott Morrison has cut short a Hawaii holiday following criticism for holidaying while his country deals with an unprecedented bushfire crisis.

Mr Morrison expressed regret for the trip, with his announcement coming shortly after two volunteer firefighters were killed and a record heatwave this week exacerbated dozens of out-of-control blazes.

Pressure had been piling up on Mr Morrison over his holiday his week, as thousands of exhausted volunteer firefighters battled blazes across the country and millions of people in Sydney choked on toxic smoke.

"I deeply regret any offence caused to any of the many Australians affected by the terrible bushfires by my taking leave with family at this time," Mr Morrison said in a statement.

Scott Morrison

He added he would be returning to Australia as soon as possible "given the most recent tragic events", referring to the deaths of the two firefighters.

Geoffrey Keating, 32, and Andrew O'Dwyer, 36, died when a tree damaged their truck yesterday as they battled out-of-control blazes south of Sydney.

Three other firefighters in the vehicle survived with minor injuries.

Members of the Horsley Park RFS at a makeshift memorial for the firefighters

New South Wales fire commissioner Shane Fitzsimmons said firefighters everywhere were grieving the "huge loss" of the two men who were caught in "the worst imaginable set of circumstances".

"(They) simply went out, doing a remarkable job, like all their colleagues, and like they have done year-in, year-out, and to not be coming home after their shift is a tremendous grief," he said.

The fire that they were battling remained out-of-control and conditions were due to worsen tomorrow, leaving no time for their colleagues to grieve as conditions worsen.

More than 100 fires are burning in New South Wales, the most populous state in Australia with Sydney at its capital.

A fire truck at a bushfire near Bilpin, 90km north west of Sydney

A state of emergency was declared in New South Wales yesterday because of the "catastrophic" conditions.

Aside from New South Wales, more than 70 fires are raging across Queensland state to the north. Bushfires are also burning in Western Australia and South Australia.

At least three million hectares of land has been torched across Australia in recent months, with eight people killed and more than 800 homes destroyed.

Australia endures bushfires every year but they began particularly early this season, lasted longer and have been far more intense.

Scientists have attributed this in part to global warming, although Mr Morrison's conservative government has been reluctant to link the fires with climate change.

Protesters holding placards demand climate action outside of Kirribilli House in Sydney

A series of record temperatures this week has further raised the alarm about global warming, as well as worsened the bushfire crisis.

Australia endured a record national maximum temperature of 41.9C on Wednesday.

This was a full degree higher than the previous record set just one day earlier.

Until this week, the record high had been 40.3C in January 2013.

In some parts of Australia the temperature neared 50C, and the heatwave was expected to continue into the weekend.