Without adjudicating on the issue, if ‘in writing’ was in the form of a formal and properly written Letter of Complaint per the Australian Consumer Law, including a reasonable date for their response it would be time to engage Fair Trading with a CC to the Managing Director or Owner, excepting Billi appears to be a private company that does not make it easy to identify their most senior management; you could address Mr. Dean Cook identified as head of sales, Australia per LinkedIn as well as to the ‘Customer Support Manager’ who is not explicitly identifiable.
My much earlier post in another topic may assist if you read it through to confirm whether your letter covered what is required to get attention.
