Emergency services have freed a fire truck that was caught in a hole for 10 hours and teetering on the edge of a cliff in Bilgola on Sydney's northern beaches.

The fire crew was responding to reports of flooding due to a burst water main about 4:00am when the vehicle hit what they thought was a pothole on The Serpentine Road.

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Crew members said they scrambled out of the truck when the ground opened up beneath them.

After first using a crane to lift the 14-tonne truck out of the hole, winches and airbags - normally used to lift train carriages - were brought in to help move the truck out of what police called a sink hole.

Fire and Rescue NSW Superintendent Kel McNamara said when the truck became stuck, the crew immediately ensured people in the area were safe.

Earlier, Supt McNamara had said the truck was stuck in a three-metre deep sink hole, tethered to a tow truck at a 45-degree angle on the cliff.

What is a sinkhole? A sinkhole is a geological feature developing when limestone or a similar rock type is dissolved by water and collapses.

A sinkhole is a geological feature developing when limestone or a similar rock type is dissolved by water and collapses. Either rain or groundwater can dissolve the rock, causing erosion that can lead to underground caverns.

Either rain or groundwater can dissolve the rock, causing erosion that can lead to underground caverns. It can take a long period of time for a sinkhole to form.

It can take a long period of time for a sinkhole to form. Significant rainfall in an area can add to the development of a sinkhole.

Significant rainfall in an area can add to the development of a sinkhole. Extreme drought and flooding can also impact the soil structure which can result in faster forming sinkholes.

Extreme drought and flooding can also impact the soil structure which can result in faster forming sinkholes. Underground water will impact soil movement below the surface that is unseen.

Underground water will impact soil movement below the surface that is unseen. Sinkholes can also be caused by human intervention such as removing groundwater from a location. Read more on ABC Science.

Police had feared the hole could open up further, dislodging the truck and causing it to roll onto houses further down the embankment.

At the bottom of the cliff the burst water main caused huge amounts of damage to at least five homes.

Backyards have been covered in mud, retaining walls have been destroyed and houses have filled with mud and silt.

State Emergency Services (SES) workers and engineers were called in to assess if there would be any further landslips.

A police spokesman said there had been similar incidents on The Serpentine Road and residents were aware of the issue.

"The people who have been evacuated self-evacuated, so they're looking after themselves," he said.

"There's one couple we are providing some assistance with, using the SES."

Sydney Water did not know what caused the burst main and shut off the water.