A bushfire over 80 hectares in size burning in Sydney's Northern Beaches has been downgraded by New South Wales Rural Fire Service (NSW RFS) from Emergency to Advice.

Firefighters with both the NSW RFS and Fire and Rescue NSW were battling the blaze, near Oxford Falls, today until it became out of control.

By mid-afternoon, the fire was upgraded to an emergency warning, with the blaze burning east towards Cromer Heights.

At the time, there were Fire and Rescue NSW fire trucks in attendance, with 30 firefighters assisting the NSW Rural Fire Service.

However, by 5:30pm the fire was downgraded to the Advice alert.

According to the NSW RFS, residents in the vicinity of Pinduro Place, Jersey Place, Maybrook Avenue, Kirrang Street, and Lady Penryhn Drive should be aware of ember attack.

RFS Inspector Ben Shepherd said the fire was caused during a hazard reduction, with embers being carried by gusty winds past containment lines into coast heathland. 

"But look, fortunately, at the moment, no reports of any property loss, no reports of any property damage, but still a number of hours of work to do before we can get this fire contained."

Mr Shepherd said conditions this evening would assist firefighters in getting control of the fire.

"As we move into this evening, conditions will become more in our favour and hopefully we can get this fire under control," he said.