It’s a pity you’re going North in Oz. There’s not too much in the way of trail centres but Canberra has some good organised trails and there are some crackers around Melbourne (the You Yangs and the Wombat trail being the best for me with Lysterfield being the most trail centrey and Mount Buller being the most Alpine red bullness place). There are some secret squirrel trails on Mount Tambourine near the Gold Coast but, to be honest, you’d be lucky to find them, even if I tried to give you step by step instructions on how to get there.
In Kiwiland, there’s almost too much to choose from. Rotorua, Taupo, Wellington, Queenstown and Wanaka all have great trails (having ridden them all on my trip over there recently, I can recommend all of them). There are good bike hire places in all of these towns, so hunt around, get the bike you want to ride and get out there!
Enjoy your hols.