How much can I charge in overdue account fees and/or interest?

I am not an authority on Australian law but, When I ran my own business here in the UK I put a 30 day notice on bills with a proviso that extra costs would be involved if the bill was not settled within this time. Whatever my costs to me were after that time is what was added to the bill. if that didn't work then I would get my solicitor to write to the client explaining that their costs would escalate if they didn't comply with the conditions, that usually worked. Failing all this the the small claims court was the next step. Just a tip! Don't promise a discount for early settlement, because they generally go over time and still deduct the discount so you lose both ways. Hope this helps.