If you're in Oz and you buy it from Singapore, Hong Kong or the US, the chances are you won't pay any GST.
However, as the purchase is over $1,000, legally, they can charge you GST on the import, but with all the gear I've imported over the years, I've never had to pay it, and the goods have just arrived on my doorstep.
But there is still a chance.
As far as warranty is concerned, forget about Canon Australia covering the warranty, and if something goes wrong with the camera, you'll have to return it to the seller to get warranty work done on it.
If you buy from the US and have to return it to the US, there is some quaestion about whether the warranty will be valid, as Canon US has some sort of fine print in their warranties about the goods being exported out of the country and hence voiding the warranty, but you'd have to get an assurance from the seller to verify this.