Teen charged after alleged gunpoint robbery on Sydney's Northern Beaches

A teenager has been charged after allegedly robbing a service station at gunpoint on Sydney's Northern Beaches.

The boy, aged 15, allegedly entered the petrol station on The Strand in Dee Why armed with a gun and wearing a face covering on Saturday night, taking off with cash.

The nearby suburb of Cromer was locked down on Sunday after the boy allegedly evaded police during their attempts to arrest him.

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Officers armed in bullet-proof vests were seen scouring the surrounding areas looking for the boy, who took off on foot.

Sniffer dogs were deployed to the scene and eventually found the teen in a stranger's backyard with the help of locals.

He was allegedly found to have wad of cash.

"We've had the assistance of one of the police dogs and a number of local cars managed to secure a perimeter and found him hiding in one of the neighbours yards, so that was very lucky," Chief Inspector Paul Warden said.

"We'll be making further inquiries in relation to how he came to be in possession of that money.

"We are also looking at a number of other offences that may have been committed."

The teen was arrested, taken into custody and later charged.

The weapon has yet to be found.