Вот один из способов (требует меньше вызовов функций):
var myJson = {"ACT":"Australian Capital Territory","NSW":"New South Wales","NT":"Northern Territory","QLD":"Queensland","SA":"South Australia","TAS":"Tasmania","VIC":"Victoria"};
$.each(myJson,function(key,val) {
$('#mySelect').append('<option value="'+ key + '">' + val + '</option>');
});