Several bush shacks in Tasmania’s remote Central Highland are under threat from an out-of-control bushfire which has sparked an emergency warning.

The blaze in rough terrain near Black Snake Creek west of Lake Crescent was on Thursday night one of nearly 20 active fires across the state.

Nearby shacks, which have been checked by authorities, are believed to be vacant.

Efforts to fight the fire have been hampered by strong winds which are forecast to continue on Friday, when temperatures are again tipped to peak in the 30s.

Two blazes remain uncontrolled north of Hobart at Elderslie and Glenfern but they aren’t posing an immediate threat to communities.

That could quickly change, with the region’s Derwent Valley and Hobart’s east expected to reach a severe fire danger on Friday.

A total fire ban remains in place for the state’s south on Friday, before a cold front and rain are tipped at the weekend.

Meanwhile, Sydney residents are again being warned of health dangers as dense smoke covers the city and much of northern NSW.

Seventy fires were burning across the state, 44 of which were uncontained on Thursday night. Most of NSW is facing high to very high fire conditions on Friday.

Fires burning on the state’s mid-north coast, which are believed to have killed at least 350 koalas, sent haze south to Sydney and as far as Bowral.

The haze resulted in the Department of Environment again forecasting air quality in Sydney would continue to be “poor” on Friday.

People with a chronic respiratory or heart condition are encouraged to stay indoors.

On Wednesday, the air quality in the city and in other parts of NSW’s central and north coast regions reached “hazardous” levels.

Air quality across NSW on Thursday afternoon. Source: NSW Environment Department

The haze is due to smoke from bushfires near Port Macquarie and in the Hunter region, which has been pushed down over parts of Sydney, and as far as Bowral in the Southern Highlands.

The department’s air quality index showed parts of the lower Hunter, upper Hunter, Central Coast and Sydney’s east were rated as “hazardous”.

Within the Sydney area, Randwick, Rozelle, Chullora, Earlwood and Macquarie Park were given a hazardous rating.

Northerly winds would continue at least for Thursday, potentially bringing more smoke to affected areas, authorities said.

“It is mixing currently with fog, but we are expecting that to clear a little across the day. But it may not completely clear for another 24 hours,” the NSW Rural Fire Service said on Thursday morning.

Hot & windy conditions ahead of a strong cold front will bring elevated fire dangers to southeast #WA & #SouthAustralia, moving into #Victoria on Friday.

Cooler & showery conditions (with possible storms) will follow behind the front. Latest forecasts: https://t.co/Na8f3vyfeN pic.twitter.com/8PcJxXa9Yu

— Bureau of Meteorology, Australia (@BOM_au) October 30, 2019

There were 60 fires burning across NSW on Thursday. The RFS said 38 were uncontained.

Rain is expected across the state on Sunday, offering partial relief. A cool change is also forecast for South Australia, Victoria and Hobart.

There are no total fire bans in place in NSW for Thursday, but the greater Hunter, greater Sydney, Illawarra/Shoalhaven, far South Coast, Southern Ranges and Central Ranges areas are under very high fire danger.

-with AAP