Restaurant staff noticed leaks from the roof of their bathroom and storeroom about 7pm, their peak dinner service time, after the vandals left fire hoses running upstairs.

"I walk into our foyer and it's all flooded in the foyer,'' La Porchetta owner Sandro Discipio said. ''Then I go upstairs and have a look and someone's broken in, smashed all the plaster and all the walls inside the offices there and left the two fire hoses running."

After turning off the hoses, calling police, and turning off the power, Mr Discipio had his chefs cook by torchlight for diners already in the restaurant, as he turned away customers at the door and cancelled phone orders at the popular pizza restaurant.

Mr Discipio spent Thursday racing to make repairs repair so the restaurant could resume trading.

Senior property manager Eleni Notaras, of LJ Hooker Commercial, said the damage in the large upstairs office looked bad, but she was able to see the positive side of things.

"For the size of the unit, it could have been a lot worse," she said, explaining most of the damage was caused by the fire hoses. "They turned them on and went to town … flooded probably 40 per cent of the floor.''

Mr Discipio said he was "disheartened", having lost stock as well as trade while working with builders and electricians to repair his restaurant.

"These idiots do things and they don't think about what they're doing because it all comes out of someone else's pocket," he said.

Police would like to speak to witnesses who may have seen anything suspicious at Four Seasons House on Wednesday night. Contact Crime Stoppers on 1800 333 000, or via www.act.crimestoppers.com.au.