There really isn't an issue in it being an Aussie ref. There are really only 3 national sides of note, and the NZ players play in Australia. They've picked the top ref, and he happens to be Australian.
They have probably picked the best referee, but a lot of people would disagree it's not an issue or doesn't give the Aussies an advantage.
This is because NRL & Superleague refs tend to apply the rules differently or more strictly/leniently within certain aspects in the game.
For instance, Superleague refs tend to be more strict at the 'play-the-ball' area and penalise more for lying on in the tackle/ trying to delay the play.
Therefore Superleague teams gear a lot of their game around getting quick play-the-balls, as it gives the defending side less time to get their defensive line 'set' and organised, which usually leads to more line breaks (or penalties).
In contrast Aussies refs are more lenient, which means the defending team get more time to get their defence organised, making it harder for the offensive team to make 'breaks'.
So in essence our players spend all season playing a certain way, but then have to adapt for one-off games when they get reffed by an Aussie, putting them at a dis-advantage.
You could then ask 'why don't SL refs ref similiar to the NRL?', and the answer is that SL games are generally considered more open and exciting games, compared to the NRL.
Some NRL teams (like the Melbourne Storm) are quite mechanical and rigid in their play style, where their whole game is based on five big carries, a good kick & chase, then pressuring the opposing team to make a mistake near their own goal line.