An Australian family has been left stranded in Bali after a fire at a popular Seminyak restaurant.

Police have seized their passports while investigating claims that balloons they brought in for a birthday celebration may have sparked the blaze.

Caitlin Speedy from Western Australia was celebrating her mother’s 70th birthday at Chez Gado Gado when the fire broke out.

Ms Speedy said she ran towards the fire to make sure everybody was safe, with her 17-year-old son suffering burns and requiring hospital treatment.

The family believes the balloons, which they understood were filled with helium, may instead have contained highly flammable hydrogen.

However, Indonesian authorities have not established that the balloons caused the fire, with an investigation into the incident continuing.

Despite the family being treated as witnesses rather than suspects, their passports have been taken by police, preventing them from leaving Indonesia.

Ms Speedy said the documents were initially returned by Kuta police, only to be confiscated again when the family arrived at the airport. They are now seeking assistance from Indonesian immigration authorities to resolve the situation and return to Australia.

Ms Speedy and her partner, Jye Wals, have questioned why they are being prevented from leaving Bali when they have not been charged with an offence.

“I don’t think we should be held here,” Mr Wals told 7News. “There were 300 people in the building, like everyone else, essentially, is a witness.”

The fire caused significant disruption at the restaurant, although no serious injuries have been reported beyond the burns suffered by Ms Speedy’s son.

Police are continuing to investigate what caused the fire and whether the balloons were filled with helium or another gas.