Police believe a wheelie bin was set alight outside the back of the premises before 12.25am yesterday morning, causing up to $50,000 damage to the building.
A police spokeswoman said the fire was deemed suspicious and the Arson Squad was investigating.
West Perth resident Mrs Davies said police called her at 12.30am.
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READ NOW"When they told me it was $600,000 I visualised most of the shop was gone but as long as my bride's dresses are safe, that's all I thought about," she said.
"It's dreadful; I would have been a mess if we'd lost customers" dresses."
Hob Nob, which has been at Beaufort Street for nine years, did not have electricity by mid morning yesterday as a result of the damage.
Mrs Davies said she lost 10 new dresses worth $10,000 in total.
"I'm so lucky we're still able to open for business," she said.