Andy,
I can’t answer for NSW, but in the ACT, our Act specifies 10% simple interest to be charged on overdue levies. This statutory default can only be changed to an amount less than 10% or up to 20% by special resolution. Perhaps your act has a similar provision.
By way of comparison, our levy notices include a statement:
“After levy due date first letter $0.00
+14 days after second letter $55.00
+14 days after final letter $110.00
+7 days after Notice to debt collection agency $250+legal fees”
These service charges by the managing agent are included as terms in our management agreement, which are the managing agent’s default position. Since the management agreement is agreed by general meeting resolution, the OC has, in effect, resolved to agree to these service charges for overdue levies on top of the interest we are required to charge. In practice, a little bit more time tends to be left before getting heavy but our OC has not often had problems with unpaid levies.