The Department of Environment's air quality index showed parts of the lower Hunter, upper Hunter, Central Coast and Sydney's east were rated as "hazardous" on Wednesday evening for either visibility or particles.

Within Sydney, Randwick, Rozelle and Earlwood were among areas rated as hazardous while Macquarie Park, Chullora and Bringelly were among areas rated as having very poor air quality.

The department has forecast air quality in Sydney for Thursday as "poor" due to particles, warning it is unhealthy for sensitive people and could cause symptoms - especially in people with heart or lung disease.

Smoke haze from intensifying fire conditions on the state's mid-north coast and the Hunter region has been dragged down towards parts of the Central Coast, a Bureau of Meteorology spokesman said.

"We have seen quite extensive areas of smoke haze that are being pushed throughout the state in the east," he told AAP on Wednesday.

Smoke from fires near Port Macquarie has been pushed down over parts of Sydney and as far as Bowral, he said.

Fires over the western part of the divide have been pushed down to Wagga Wagga.

Northerly winds will continue into at least Thursday with the potential to bring more smoke over the affected areas, the spokesman said.

A bushfire which continues to burn in the area of Lake Innes and Lake Cathie, south of Port Macquarie has burned more than 2425 hectares, including an important koala breeding ground.

Port Macquarie Koala Hospital president Sue Ashton told AAP the loss was a "tragedy".

"The beauty of this particular population is that it's so genetically diverse that it's of national significance," she said.

"A lot of the koalas are being mixed and cross-bred now so to lose a large part of that population is very devastating."

Ms Ashton said while the hospital's trained rescuers would be unable to search for any surviving koalas until Thursday or Friday, she feared the worst as koalas were "terrible" at coping in a bushfire scenario.

On Wednesday evening the fire, which has been burning since Saturday and is suspected to have been caused by a lightning strike near Port Macquarie, had been downgraded to advice level.

Residents near Long Point Drive and Cooinda Place in Lake Cathie should put their bush fire survival plan into action, monitor conditions and take advice from firefighters in the area, NSW Rural Fire Service said in a statement.

Residents in Lake Cathie, Long Point or Bonny Hills should monitor conditions.

NSW RFS spokesman Greg Allan said crews will continue working towards containing the fire as soon as possible, strengthening containment lines and undertaking backburning where conditions allow.

"We're pretty much putting all the available resources we can into controlling this fire," Mr Allan told AAP on Thursday evening.

Hundreds of firefighters have worked to fight the fire with assistance from other agencies and aircraft, he said.

NSW Fire and Rescue also undertook hazard reduction in North Sydney on Thursday.

NSW Health reminded people that children, older adults and people with heart and lung conditions were most susceptible to the effects of air pollution and excessive smoke.

"It's really important that people who have pre-existing heart or lung conditions like asthma are alert to the risk of the smoke and if you can see smoke or smell smoke it's a good idea to try and avoid outdoor physical activity," the department's director of environmental health Dr Richard Broome told AAP on Thursday evening.

Australian Associated Press