Have you asked an Australian Agent to price match? It might not be possible, but if you don't ask, you don't get!
Please don't for a second think that the difference in figures quoted by the OP are made up of margin the TA makes, it's most certainly isn't, the average margin on a coach tour is anywhere from 9-14%
From experience I've asked tour companies to match an OS price before without success, their attitude is that the US (or UK as it was at the time) agents aren't meant to sell to individuals outside of the US or UK, but that obviously a blind eye is being turned by someone somewhere along the way.
I have no issue with someone saving money when they can, but you need to realise that there are risks involved, mainly that there's no consumer protection for you and there's no provision for cover under the travel compensation fund.
You may also find (and I'm only speculating on this last point) that insurance companies who provide coverage for travel provider insolvency may not cover it if your US based TA of choice either goes belly up or worse still runs off with your money.
When you book with an AU based TA, the majority of them operate audited trust accounts, so you know your money is safe and that if anything goes wrong you have the TCF to claim under.
Not trying to put you off, just educating on a couple of aspects of booking abroad that are often overlooked in the hunt for a bargain.
TG