I'm sorry, but it really looks you have been scammed.  A high priced item ($2,500) paid by bank deposit to an eBayer with little or no feedback because their PayPal was full = SCAM

The best you can probably do is report this to the Qld Police who handle 'online fraud' from all Australian states apparently.  Someone else will come along and give you the web address.

Perhaps the best advice I can give you, and I will probably get a slap from eBay for saying this (as well as this post being removed), is always buy those high priced electrical goods from a real store where you get served over the counter.  You get all the paperwork for warranty AND if something goes wrong with the item you take it back to the store.

Can Aus post tell me where the item is in transit (if it is) if I prove that I bought the item?

without a tracking number the answer is NO.  You may be able to prove you paid money to someone by bank deposit and show them the eBay listing but this means nothing to Australia Post.

Bottom line . . . . the item most probably has not been sent to you!

What you need to do is to report this from this link:

http://www.police.qld.gov.au/programs/cscp/eCrime/internet/report/