Ooh an Aussie supplier! I've seen a few decent board-game related designs online, but they're always manufactured or shipped from the US, so postage is the killer (I don't feel like paying a total of AU$40+ for a single t-shirt).
More than happy to hear that you'll be giving this endeavour a shot!
I'd also advise that designs shouldn't be in breach of anyone's trademark or copyright - so many of those t-shirt designs aren't sending money to the owners of the IPs (mashups especially) and I feel a little dirty if I buy from them. However, you can usually get around this by *hinting* at the original product without actually mentioning it or breaching any trademarks.