A fire at Corryong in Victoria's northeast could merge with the Tumbaruma fire on NSW's southern border, prompting warnings for residents in the path of the fires to evacuate immediately.
"There is still a window for people to leave and we encourage them to do so," New South Wales Premier Gladys Berejiklian said on Thursday afternoon.
NSW RFS Deputy Commissioner Rob Rogers says parts of NSW will reach catastrophic fire danger level on Saturday.
"Whatever the rating is, it’s going to be a pretty bad day."
Six southeast NSW districts and the ACT have extreme fire danger rating on Saturday.
Severe conditions are forecast for Sydney, the Hunter and central rangers.
Temperatures are forecast to reach 45 degrees Celsius inland and up to 44C on the coast. Sydney's outskirts could hit 46 degrees Celsius.
Relief from a cool change will not hit NSW until late in the evening, possibly even after midnight Saturday.
Hope for no further loss of life
Mr Rogers says that increases the fire danger.
"I was talking to the bureau earlier and they are suggesting it could be delayed further, the change, and every time it is delayed, the fire danger gets higher and higher, so we will be watching closely."
At least 449 homes have been destroyed in the state since Tuesday and Mr Rogers says he expects more will be damaged come Saturday.
"We are unfortunately very likely to lose homes tomorrow but we will be very happy and call it a success if there are no lives lost.
"That is a single focus tomorrow and we need the community to have the focus as well, and then obviously, call for help when you need it, 000, life-threatening only."
Heat, wind, lightning add to NSW fire fear
Temperatures surpassing 40C are set to combine with dry lightning strikes and wind to add to NSW's bushfire nightmare over the weekend.
But a number of days of respite are expected to follow.
Saturday's forecast paints a grim outlook for parts of the state - particularly the NSW south coast - already battling scores of uncontrolled deadly bushfires as residents flee their homes and holiday-goers cancel plans.
Heat is set to quickly rise on Saturday before a change sweeps over the state, Bureau of Meteorology acting NSW manager Jane Golding said.
"In short, we've got a long hot day to get through first with some really dangerous fire dangers," Ms Golding told reporters on Friday.
"That cold front bringing that southerly change, we're expecting that not to reach the far south coast ... until late in the day, to move through the Batemans Bay region early evening and come through Sydney about midnight."
Two-day total fire ban
The fire danger will reach extreme levels in some areas and the forecast late cool change is due to bring thunderstorms and lightning.
NSW faces a two-day total fire ban from Friday, the third seven-day state of emergency in as many months and demands for those across the state's south coast and Snowy Valleys to evacuate immediately.
"The chance of new fires from lightning tomorrow is high," Ms Golding said.
"The main area we're concerned about is an area stretching right from the South Australian border to the central and southern slopes, western slopes of NSW."
Relief expected from Sunday
On a brighter note, the Rural Fire Service said weather conditions around NSW would ease for a number of days from Sunday.
Southern NSW may experience some showers on Sunday and Monday.
"There's potential for some elevated winds later in (next) week but no big heat spikes because of that hot air mass, there's some more moist air brought into the west of the country helping to flush out that hot air," RFS deputy commissioner Rob Rogers told reporters on Friday.
"There's some potential showers Sunday, Monday, particularly in Victoria that potentially might push up to the south of NSW. That'll all be welcome."
NSW Health, meanwhile, warned people to remain cautious about air pollution, with the Sydney basin likely to endure smoke haze on Saturday.
The elderly and those with lung and heart conditions have been advised to remain indoors and avoid exercise.
More than 130 fires are burning across NSW, with almost 70 uncontained and more than 2000 firefighters in the field.
Disaster relief payments are available for some residents.
Prime Minister Scott Morrison, who faced some angry victims while visiting bushfire-ravaged Cobargo, defended his government's response to the crisis.
Police said they are investigating "all aspects" of the south coast fires, including the eight deaths and the cause of the blazes. Mr Rogers previously said the majority of fires were sparked by lightning.
In Sydney, the Royal National Park will be closed from Saturday until further notice, as will Heathcote National Park and other western Sydney parklands.
NSW hotspots to avoid:
* the Shoalhaven from Burrill Lake to Nowra
* Batemans Bay to the Victorian border
* the Snowy Valley areas including Tarcutta, Batlow and Tumbarumba
* the Snowy Monaro area including Adaminaby to Cooma, Jindabyne and Perisher
* around Sydney's fringes, including near Green Wattle Fire at Mittagong and west of Penrith where the Grose Valley fire is still burning uncontrolled
