I don't think you're account specific limited to AUS $900, as according to
http://www.thegreek.com/bet-limits, it seems that it is the standard bet limit for everybody, in fact it states US $500 is the maximum stake or to win amount (whichever is lower), and US $500 is currently about AUS $658, so the limit for you in AUS $ should probably be AUS $658 rather than AUS $900, it's probably just that The Greek haven't updated the exchange rates to reflect their limits recently. So it seems that the AUS $900 limit you currently are getting that you aren't too happy with is actually about 25% better than you should actually be getting, so the limit will most likely get a lot worse still for you when The Greek get round to updating their exchange rates to reflect their limits in different currencies.
I'm not sure if The Greek offer the option to rebet the line after a set period of time, but if they do, that could be an option for you, or if they don't, you may be able to ask them to allow this feature for you, but obviously each max bet on their lines may mean your next bet could be at lower odds.
I've also read a couple of times in the past that on occasion The Greek may increase limits for some players on request, so it may be worth you contacting The Greek to see if they can raise your limits.