That's very poor customer relations, but then again, can we expect anything better from the TAB? They are so inflexible which can very easily lead to justified anger by their customers, as proved in your situation. Maybe if each case was judged on its merits...?

There is a good reason why they have imposed a time limit to cancel bets, and that is to avoid market manipulation which in your case would seem extremely unlikely. And we can cancel a bet within a paltry five minutes of it being placed at any of their outlets, so why then do they deny the online punter this "generous boon"? I suppose there's a good reason but I can't think of one.