Firefighters discovered more than a dozen young people living in shipping containers and caravans when they were called to a fire at an industrial complex in inner-city Sydney overnight.

Eighteen people, believed to be of an Asian nationality, were found inside the containers and vans at the back of a large factory compound in Alexandria.

Firefighters at the site in Alexandria. Credit:Peter Rae

Electricity had been wired to the units and they appeared to be well established on the site, NSW Fire and Rescue Acting Superintendent Wayne Phillips said.

Emergency crews were called to the fire at a public works building on Burrows Roads about 1.40am on Wednesday.