of course it doesnt have to be the same eagle. the only important ting is that it's a mexican eagle with a snake in it's beak. I only chose that one because of the ones i found on the mexican flags on wikipedia that one looked the best, but I expect the artist to do better than that. By the way, do you know of any other Australian symbols other than the southern cross, the commonwealth star, the kangaroo and the emu? I honestly don't think they are cool, and yet I don't want to leave australia out of the tattoo.
cheers
EDIT: I'd never consider getting the flag of any country done because they are prone to change in a single lifetime. and especially the Australian flag is in my opinion bound to change soon, so then the flag might no longer represent the nation, but instead the governments in the era of the flag. for example if the current flag changes it will obviously loose the Jack, so anyone with the old flag tattooed would be suspected of being a loyalist of the british crown, which I've heard doesn't go down well with most Australians :)