Latitude Financial fears the cyber attack on its system could be worse than first thought and is not taking on new customers.
Some 330,000 customer records and identity documents were stolen last week.
In an announcement to the ASX on Monday, the financial services company warned it was bracing for the full scale of the attack to widen.
"We are likely to uncover more stolen information affecting both current and past Latitude customers and applicants," it said.
Hollywood star Alec Baldwin once appeared in advertisements for the company which offers loans, insurance and credit cards.
It is not taking on new customers including through retailers it has agreements with such as David Jones, JB Hi-Fi, The Good Guys and Harvey Norman.
All its platforms remain offline but the company is working to restore them in coming days.
Latitude said about 96 per cent of the personal information stolen was driver licences or driver licence numbers, with the rest being passport and Medicare details.
It promised to work with authorities to replace identification documents if necessary, at no cost to customers.
It's believed the hack originated from a major vendor and criminals accessed an employee's login credentials, before stealing customer information from two service providers.
The company urged customers to remain vigilant about their online safety and said it would never contact anyone requesting their password.
It said its insurance covers cyber security risks but is still working out the total cost of the attack, which is now under investigation by the Australian Federal Police.
Latitude Financial Services chief executive Ahmed Fahour said he understood frustration directed towards the company.
"I sincerely apologise to our customers and partners for the distress and inconvenience this criminal act has caused," he said in a statement.
"I fully understand the wider concern that this cyber-attack has created within the community.
"Our focus is on protecting the ongoing security of our customers, partners and employees' personal and identity information, while also doing everything we can to support customers and applicants who have had information stolen."
Last month Latitude revealed it would end its buy now, pay later scheme in Australia and New Zealand, which has been used by about half a million customers.
