For all practical purposes, Polaramine (which is sold without prescription in Australian pharmacies) is the same as Piriton. The former contains dexchlorpheniramine, whereas the latter contains chlorphenamine maleate. They're both based around chlorphenamine though, and are effectively the same drug.
I certainly wouldn't try to get any form of drugs past Australian customs without loads of paperwork though. They're incredibly strict. (I once sent a copy of the England Cricket Annual to a boy in Australia. It never got to him though because I'd failed to fill in a form correctly).
While you'd be permitted to take Piriton into Australia for your own personal use if you were visiting Oz, you might easily fall foul of the Ozzie 'Therapeutic Goods Administration' rules if you sought to post it into the country.