Not exactly. You have to get one if you're required to register for
GST (which, as you correctly stated, is if your turnover is > 50K),
but "hobbyists" aren't entitled to an ABN because they aren't
"carrying on an enterprise". The largest down side of not having an
ABN is that some businesses are reluctant to deal with you since you
can't issue them with tax invoices (which, by definition, must include
an ABN), however in the circumstances that Ron was describing, where
the sales are retail, this probably won't be much of an issue.

Of course, the murky question, as Ron alluded to, is where the line
between "hobbyist" and "business" is; and unfortunately (aside from
the 50K turnover rule) it isn't black and white.

Ron, you may find the following link ("Am I in business?") of
interest:

http://www.ato.gov.au/youth/content.asp?doc=/content/66884.htm

Actually, getting the Tax Office to consider that you ARE in business
may be the hard part unless you're making a substantial proportion of
your income from your photography. Why?

Because if they assess you on your photo income (which I'm sure they'd
LOVE to do, being tax vampires and all), they ALSO have to allow you
to claim deductions on any expenses incurred in earning that income.
That would include things like framing... but also depreciation on
your camera gear. So I could, in theory, sell a couple of hundred
bucks worth of photographs per year, and offset depreciation on a new
5D and a couple of L series lenses against that and, of course, my
other income. Which, in my tax bracket, would be a verrrrry nice
little rort indeed. Except, of course, that there's no way in hell
that the ATO would accept that I was in business. But hey, I can
dream. 8^>

If you're in doubt, the safest way is to approach the ATO and get a
written ruling on your own circumstances. At least that way your
backside is covered.