Delay interest is interest that accrues on overdue invoices. It must have been 14 days before you can demand, or be demanded, delay interest on a reminder or a collection notice.
When does delay interest start running?
The interest starts running from day one after the payment falls due, but cannot be demanded until 14 days have passed, so in practice you cannot be demanded delay interest if you, for example, pay 7 days late.
The interest rate is defined by the Australian Government. At the same time, the size of collection fees (salaries) that can be demanded on unpaid invoices is also defined.