That depends on what kinds of drugs and supplements the horse was on. When I ship horses in from the USA I just Know they are loaded full of drugs. They are supposedly for "theraputic" reasons which makes them legal in the USA. The fact that they DO in fact enhance performance is seen as a happy but unavoidable side effect. Same story for the vitamin cocktails or PH solutions. Those horses I give at least 4 months, but the avarage seems to be 6 months to let down and then regain the weight they loose after coming off all the "theraputic" agents. My Mare Kaolin took allmost a year, I sent her to live with a freind who worked for Ian Miller. Even SHE had a hard time putting the weight back on that mare.
The drugs rules are a lot stricter in Canada so letting them down takes a matter of 6 weeks to 3 months. We seem to have less issues with horses with such thin soles as well. Sometimes the breeding makes a difference. Mr. Prospector horses, especially the Carson City line are such solid minded horses and have decent legs that they don't need much let down time before they can start work.
I have no idea what kinds of theraputic agents are du' rigor at Austrailian tracks or what your drug laws are so I have no idea what your letdown time is. The rule is however the more lax the drug laws the longer the letdown.
We had a Stud come in from South America, he took allmost 2 years. We had to keep putting him back on supplemets and small doses of steroids as he would not eat or keep weight without them. OMG was he a metabolic nightmare. He was Eso, born in 1971. Now deceased.