Motorists are advised to avoid a busy Northern Beaches road after a demolition mishap left a building at risk of collapse in Balgowlah on Tuesday afternoon.

Engineers have been asked to inspect the former Porter's Liquor building at the intersection of Sydney Road and Woodlands Street, after an excavator knocked through a wall and struck an adjoining commercial property, a Fire and Rescue NSW spokesman said.

A building collapse in Balgowlah has caused peak-hour chaos on Tuesday. Credit:Nine News

The adjoining building, a gym called The Booty Parlour, suffered structural damage.

Real estate agent Jaala Cusack, who works about 60 metres from where the incident occurred, said she heard sirens in the area about 5.10pm and didn't think much of it.