Firefighters contain 'out of control' bushfire on Sydney's northern beaches

Firefighters have finally managed to contain a bushfire on Sydney's northern beaches that started after a hazard reduction burn off got out of control.

At 1.20pm the fire in bushland near Belrose broke containment lines and by 3.30pm authorities deemed it out of control.

Eighty firefighters and two water bombing helicopters attempted to contain the fire but within in an hour the number of firefighters on scene grew to 155 as the fire continued to spread.

Teams managed to finally get the blaze under control about 6pm as cooler conditions moved in.

What began as a 15ha hazard reduction burn is expected to reach at least 70ha by the end of the day.

No one was injured during the fire.