An air quality alert has been issued for a smoke-covered Sydney due to the Rural Fire Service (RFS) taking advantage of favourable weather conditions to conduct hazard reduction burns.

Smoke covers Sydney during hazard reduction burns this morning. ( Supplied: Neil Saunders, Twitter )

The RFS said the smoke over Sydney had blown in from "controlled burns being undertaken in the Hawkesbury region".

"These planned burns are an important part of preparing for and managing bush fires," an RFS statement said.

"Hazard reduction activities will continue while dry conditions remain.

"There is likely to be further smoke impact throughout Friday, Saturday and Sunday."

Earlier, the RFS had said the areas particularly affected by the burns would include the Blue Mountains, Hawkesbury, Central Coast, Hunter Valley, Wollondilly, Queanbeyan, and Hornsby.

The Bureau of Meteorology (BoM) has issued a poor air quality forecast alert for the Sydney region.

"Particle levels are forecast to exceed national air quality standards for today," the BoM website said.

Smoke covered the Sydney skyline for a number of days after controlled burns were conducted earlier this month.

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Sydneysiders were treated to a week of spectacular sunrises and sunsets after hazard reduction burns and low clouds created perfect conditions for a stunning skyline.

NSW Rural Fire Service Commissioner Shane Fitzsimmons said more controlled burns were scheduled to take place in the coming week.

"Favourable weather conditions have again provided us with that welcome window of opportunity to continue our burning program this week," Commissioner Fitzsimmons said.

"The weather can often prove to be a significant challenge for this type of work, so we are grateful that the current conditions have provided agencies with an excellent opportunity to complete some critically important burns."

Sydneysiders advised to avoid outdoor activities

Commissioner Fitzsimmons said while there may be a lot of smoke in some areas, people are reminded to only call triple-0 if they see an unattended fire.

Sydney has been covered in smoke during back-burning by the RFS. ( Supplied: Kristina Wogowitsch, Twitter )

Residents near the controlled burns are encouraged to take sensible precautions such as avoiding outdoor activity in affected areas, keeping doors and windows closed, and bringing washing inside, he said.

Motorists driving through smoke affected areas should slow down and switch on headlights.

Susannah Hayward, of Cremorne, told ABC 702 Sydney "mega back-burns" had blighted the Mother's Day weekend, making the air "third-world toxic".

"I'd love to know who is authorising this and who thinks it's okay to blithely tell Sydneysiders simply to write off their day and stay indoors to avoid breathing difficulties," she said.

"I'd also love to know how many more of these lost days we can expect in the near future.

"I've lived in Sydney for 28 years and I don't ever remember hazard-reduction burns being done in autumn, or being done on such a large scale that the air in the Sydney basin is completely toxic for hours at a time."