I have a bill of $150. I'm unable to pay the full amount before the due date however I can pay most of the bill.
If I pay $101 I will have $49 still owing. (I can pay the $49 a week after the due date.)
The website at optus.com.au/latefees has a table of "unpaid amounts" and associated fees starting at $50.
Live Chat seems to think there is an unpaid fee but isn't sure how much it is. I also don't trust the answers they provide.
If I owe $49 past the due date is there a late fee and how much is it?
As you've noted the late payment fees do appear to start at $50. If there is a fee for $0 to $50 (and I can't find any mention of one) then obviously it will be $1 to $15 (max).
I think you're safe at $49 (although make it $48 just to be safe if you can). The CS people can be a bit variable in the advice they give. I suspect your guy was probably refering to the fact that
"To arrange a payment extension, click here. Note: You may (will) still incur a late fee after agreeing to a payment extension."
PS Probably not needed but you can arrange an automatic extension via the app