Questions over how Sydney hazard reduction burn got out of control

There are serious questions over how a hazard reduction burn on the Northern Beaches in Sydney became an out of control bushfire.

It threatened properties at Beacon Hill and Cromer Heights, and is expected to keep burning for days.

The flames kept residents on edge today as the blaze burned in bush at Oxford Falls.

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Rob Butera said his family left as he stayed put.

"My wife and kids ended up going to the in-laws and I stayed back, took care of the house," he said.

The out of control fire was started yesterday by embers from a hazard reduction burn, which blew outside of the control zone.

It put crews on the back foot with residents warned it was too late to leave.

Inspector Ben Shepherd from the Rural Fire Service said while "problematic", crews planned for such things.

"No one is eating themselves up more than the crews that were here actually trying to do this burn yesterday," he said.

"Over here on the Northern Beaches, and in many coastal parts, we have what we call coastal heat. So it's a very fine fuel. It burns very, very rapidly."

The hazard reduction burn was planned for 25 hectares but about 140 hectares of bushland was scorched by today.

Attention has now turned to the fire line creeping west, edging towards backyards.

Back burning is the solution to stopping the fire in its tracks.

There hasn't been a fire or a reduction burn in the spot in 30 years, meaning there is far more vegetation for crews to burn through to protect homes.