Wollongong Fire Station Officer Paul Knight said they were alerted to the fire shortly after 3am when an off-duty police officer noticed smoke coming from the roof of the building.

When firefighters arrived at the building the fire was well established in the Jewellery Gallery, which is located above the Pellegrini cafe and Gossip Thai restaurant.

Mr Knight said the Jewellery Gallery's roof collapsed during the blaze and the premises were gutted by fire.

The downstairs cafe and restaurant sustained water damage but escaped fire damage.

Keira St was closed to traffic until the four fire engines and Hazmat teams left the area shortly after 7am.

The cause of the fire is not known at this stage.