So, do you write: ‘She’s in Australia.’ or ‘He’s at Melbourne.’?
Fowler, in his ‘Modern English Usage’, which, by the way, was first published in 1926 and (for my edition) last revised, with corrections, in 1974, has a little to say on this issue. He quotes the Oxford English Dictionary, that unquestionable authority on matters of English language, as suggesting that ‘at’ should be used of all towns except the capital and that in which the speaker dwells, if of any size. But he suggests that this is too narrow a usage and goes on to say that we now speak as readily of being ‘in’ any town as we do of being ‘at’ it. He goes on to say that any difference, if it actually exists, lies in the fact that ‘in’ suggests something being physically within, whilst ‘at’ does not. His given examples are as follows:
‘New College is in Oxford.’
'The new term at Oxford begins this week.’
And I have no argument with that analysis. However, it doesn’t really answer the question adequately and I wonder if it really comes down to a matter of taste.
I would never use ‘at’ to describe someone being in any town. Always preferring ‘in’ for this sort of sentence; thus:
‘I live in Driffield.’ ‘The Tower of London is in London.’ ‘Billy does his studying in Belfast.’
And I would generally use ‘at’ when referring to some place within the town; thus:
‘I shop at Driffield market.’ ‘Parliament sits at Westminster, London.’ ‘Billy studies at Belfast University.’
I hope this helps, but I suspect that both locally and globally there are colloquial takes on this usage. I suggest you adopt the form that most pleases your ear and hope that it doesn’t offend those of your readers.
Pic: A small valley in East Yorkshire, called Deep Dale.