Other than Brett & Damien is there anyone else who has purchased the product?

I would suggest those involved send an email (with read receipt) requesting a final answer as to either refund money or send product (registered mail).  If these get ignored after a reasonable time, then I would suggest it no longer is a civil matter and becomes a Police matter.  It makes no difference that victims are in S.A or QLD and the other party is in N.Z.

If you wish to go down the criminal path, make sure to KEEP ALL original emails (don't forward but make sure they are still there).  Print off copies.
Also print copies of any PM's via AP.
I would also suggest to print off the thread where replies or promises have been made.
Print off a copy (or photocopy) the bank transfer information (ie to the account money was sent too) or paypal information.

Its best to also complete a witness statement prior to attending the Police Station.  These were available on the QLD Police website (ie separate ones for each state).  Although they were for eBay, they can be modified without issue.