Hi Dogbones,
No doubt about it mate, in Tassie you are going to need a smaller kite. A smaller board is nice and will give you more control over speed, but when you are overpowered, you are overpowered and thats all there is to it.
The only time a smaller board will help significantly when majorly overpowered is if you ride in buttery smooth water and can hold a good edge at speed. This also only works if your kite behaves itself when overpowered. Some kites behave very badly when overpowered.

A more experienced rider can get control over an overpowered big kite in stronger winds more easily than a beginner. It's not just riding with an overpowered kite, it's dealing with it on launch and land and in crashes etc.
Find yourself a nice 7m to complement your 12. We have a bunch of smaller winter type kites at clearance prices, have a look see if there is something you might like. A smaller kite is more fun, quicker, lighter, and will boost you much higher than a big kite.
A smaller board will help too, and can be an addition when you are more financial. Also, what Lach says is quite true, a shorter line set of say around 20m, will have the same effect as dropping the power of your kite to that of around a 10m.
Go the smaller kite and have fun.
Good winds,