Police investigate suspicious Sydney fire

A fire at a motorcycle repair shop in Sydney's north is being treated as suspicious by police.

The building that houses the Hornsby business was extensively damaged by fire and smoke during a blaze in the early hours of Monday morning.

"No one was inside the building at the time, however, a guard dog had to be treated for smoke inhalation," police said.

A crime scene has been established and forensic specialists will investigate on Monday.

Anyone with information is asked to contact Crime Stoppers on 1800 333 000.