Hi Gary,
Not sure what part of Oz you are in, but if you're on the east coast, and it's a business you are trying to contact, try calling first thing in the morning (like 6-8am), it should be mid-to-late afternoon of the previous day in the US.
If it's a private number you are calling, you can probably get away with calling early/mid-morning our time, I wouldn't recommend any later than about 9-10am if they are on the US east coast, or midday if they are on the west coast.
If you call in the afternoon or evening (our time), you will almost certainly be waking them up in the wee hours of the morning.
