I've discovered that once I've had a story published in an Australian magazine I can try to sell it to overseas publishers. I know I need to specify what rights I'm selling (eg. First British serial rights), but is it necessary to mention it has been published here in Australia?
First you need to check which rights you've sold in Australia - if you gave them world rights, then you have nothing to sell. I'm presuming you've done that, so if you're submitting overseas you should mention that it's been published in Australia, because it's a good thing - someone else thought the story worthy of publication. But, given that we're a former colony partly populated by former Britons - a fact they conveniently forget, damn overlords, just because the aforementioned former Britons were criminals - don't expect anyone in the UK to actually care. I could spend two hours ranting about British publishers and their cavalier behaviour in respect of Australian rights, but I'm already en couleur and I should really calm down before trying to eat my lunch.