Unfortunately no personal contacts from me, but happy to suggest a dealer name or two that I've dealt with you can contact to at least start a discussion.
BTW, I'd recheck your itinerary, if you head south-east from Sydney airport, you get to my place, and then there's a whole lot of water before you hit NZ... ;-)
One of the main Sydney BMW dealers is Procycles.
They have a shop at St Peters, which isn't too far from the airport back towards the centre of the city (about 20 minutes away) on one of the main roads that you can follow out of Sydney, the Princes Highway.
They also have a shop in the northern suburb of Hornsby.
I use them because they are close to my work if I have to get something sorted urgently. Storage space might be an issue for them.
I bought my bike from a little further north (about 1 hours ride north of Sydney) at Worthington BMW ( http://www.worthingtonmotorcycles.bmw.com.au/
Great dealer and I've found them very good for service.
They are on the edge of one of our outer urban cities, so a bit harder to get to but a great place to start a trip from, right near the major freeway that runs north/south (F3).
Alternately, there's a dealer in Wollongong too, about an hour south of Sydney, but I havent dealt with them.
) a pic of the location of some of the dealers above, in relation to Sydney Int'l Airport.
Others might hopefully chime in with some alternatives, there's plenty of bike dealers around town, just not many BMW....
good luck with your trip, just remember, Jan - Feb is summer here, can get very hot (40+C), and quite often get bushfires which can affect travel options on roads.