Hundreds of Gold Coast and Sydney off-the-plan apartment buyers have been caught out by the collapse of one of the country's biggest private developers, William O'Dwyer's Ralan Group.

The developer, whose flagship project is the $1.4 billion Ruby Collection at Surfers Paradise comprising 1600 apartments spread across four towers, owes around $500 million to creditors, including Westpac Bank and Melbourne finance house Wingate.