NSW Emergency Services Minister Mike Gallagher said the extreme weather - predicted to create one of the worst fire danger days in NSW history - was now "starting to play out".
"There was a hope that something would abate this weather condition as it moves across NSW, but the fact is that it has not,'' he told reporters in Sydney.
"We are being confronted with a situation where up to three areas are deemed to be at catastrophic in terms of the fire conditions, with 90 per cent of NSW being deemed to be severe or higher.
"The far western and far south coast areas of the state have a fire danger of severe, and the rest of NSW will be at the high level.''
A total fire ban is in place across NSW, with temperatures predicted to hit 45 degrees in the state's far west and 43 degrees in Sydney - the third highest mark on record.
All NSW national parks, reserves and state forests are closed to the public due to the fire risk.
A number of emergency warnings have already been issued across the state.
An out-of-control fire has reached homes at Carlaminda in southern NSW. Residents on Tuesday morning were urged to take shelter inside as it was to late for them to leave in safety.
RFS Commissioner Shane Fitzsimmons said it was shaping up to be a long day for the thousands of firefighters in NSW.
"Right across the state we are seeing fires breaching some of their containment lines, and we've also got fire agencies identifying new fires that are starting up under these hot, dry conditions,'' Mr Fitzsimmons told reporters.
One of these is the grass fire on the NSW Riverina of Oura, which firecrews and water bombers are scrambling to put out as strong winds bear down.
About 100 firefighters and 25 trucks are protecting Oura, east of Wagga Wagga.
Temperatures in the area are set to reach 41C on Tuesday with wind gusts of 70km/h.
Two helicopters are water bombing smouldering hills near a winery and dairy farms.
Rural Fire Service officer Lesley Lemon said the fire was only a couple of kilometres away from homes.
There has been no damage to property yet.
It's believed the fire started on Saturday following lightning strikes.
Meanwhile, in southwest NSW, an emergency bushfire warning has been issued for the township of Tarcutta.
The NSW Rural Fire Service said the fire front was about 3km from properties shortly before 1pm, and residents were being urged to "shelter in place'' as it approached.
An RFS spokesman said parts of the Hume highway had been closed because of the blaze.
He said the fire was expected to hit imminently.
"It could be minutes, it could be seconds,'' the spokesman told AAP.
In northern South Australia, however, bush fires are causing little concern.
The fires, including some which have been burning since Christmas, are in remote areas and are just being left to burn.
The Country Fire Service says they pose no threat to people or property and are being monitored by satellite.
A number of other fires to the south are contained and are still being monitored by small CFS crews.
They include a fire at Finniss, south of Adelaide, at Penola, in the state's south-east and at Sevenhill in the Clare Valley.
Meanwhile, a team of New Zealand firefighters will head to Australia to help Tasmanians battling bush fires caused by extreme heat in the island state.
National rural fire officer Murray Dudfield said Tasmania fire agencies made the request for assistance last evening, asking for people with experience in fighting fires in tall timber, remote and high country areas.
Two crews of six, one group made up of Department of Conservation staff from Northland, and the other of forestry workers from the Nelson region, will fly out tomorrow afternoon. The team leader is National Rural Fire Authority Rural Fire Manager John Barnes from Christchurch.
"This team will use equipment provided by the local agencies and are likely to be deployed for at least two weeks," said Mr Dudfield.
Over the past decade Kiwi firefighters have been deployed several times to assist their Australian counterparts fight bushfires. The largest continent sent was in 2009 when 110 rural firefighters were deployed to Victoria.
"New Zealand fire authorities have built up a strong relationship with their Australian counterparts over the years and share training, knowledge, research and have agreements to provide assistance to each other when need," Mr Dudfield said.
- AAP, APNZ, AFP and nzherald.co.nz
