Australians across New South Wales and parts of Queensland are bracing for a day of "catastrophic" fire danger which will threaten lives and property. 

The NSW Rural Fire Service has warned residents to leave homes immediately, taking pets with them, in areas facing the greatest danger. 

The highest level of "catastrophic fire danger" is forecast for the Greater Hunter, Greater Sydney Region and Illawarra/Shoalhaven, while "extreme fire danger" is the warning for the North Coast, Southern Ranges, Central Ranges, New England, Northern Slopes and North Western areas. 

"Severe fire danger" is forecast for the Far North Coast, Far South Coast and Monaro Alpine, while a "very high" risk is forecast for the Upper Central West Plains, Lower Central West Plains and Southern Slopes.

To compound the threat, the Bureau of Meteorology (BOM) is predicting hot, dry and gusty winds on Tuesday followed by a cold front expected to sweep through in the early evening that could see the intense flames quickly change direction. 

Weatherzone Meteorologist Ben Domensino said Tuesday's dry "southerly buster" will cause any fires burning near the NSW coast to "abruptly change direction".

Rural Fire Service Deputy Commissioner Rob Rogers said: "I've been in this industry for 40 years, and I've not seen a scenario like this before."

"I really haven't - not when you've got all this fire and this catastrophic risk. If someone came to me and said, 'Let's do a scenario role-play' I'd say 'Let's try to keep this a bit more realistic'. It's that sort of out-of-the-box."

Up to 20,000 people are expected to join the firefighting effort on Tuesday with the Australian Defence Force on standby to help.

MAPS REVEAL POTENTIAL FIRE SPREAD

The Rural Fire Service has published a series of maps showing the potential course several fires in NSW could take in the hours ahead. 

WARNINGS OVER AIR QUALITY

Health authorities in both New South Wales and Queensland have warned residents to stay indoors if they can to avoid unnecessary exposure to smoke.

On Monday afternoon, air quality was at the lowest possible rating of "very poor" in Brisbane, Ipswich and on the Gold Coast as well as in Gladstone, Moranbah, Mackay and Townsville.

With no significant rain forecast for the next week, BOM forecaster Vince Rowlands said hazy conditions were expected to last for several days.

Dr Richard Broome from NSW Health said the best way to reduce smoke exposure was to stay inside with all doors and windows shut.

"For most people, smoke causes mild symptoms like sore eyes, nose and throat," Dr Broome said in a statement on Monday.

"However people with existing lung and heart conditions like asthma, emphysema and angina are more likely to be sensitive to the effects of smoke. People with these conditions should avoid outdoor physical activity when there's smoke around."