Righto - I was thinking of travel in the opposite direction
You might want to try to visit the Air NZ lounge (afaik AC Maple Leaf Lounge doesnt have showers or even bathroom) which is upstairs per obscure2k's directions (in fact right next to MLL). It has recently been refurbished and is very nice, including shower suites. Depending on time of day you may not be allowed in (gets very crowded when the bank of Air NZ flights leave).