AFAIK: There's a big difference between busting you red-handed and entrapment. An policeman busting you in the act of committing a crime, such as soliciting an illegal sex worker, attempting to buy/sell drugs, or anything else is not entrapment, because you were committing the crime anyway. The police haven't enticed or forced you to do something you weren't going to do.
A person is 'entrapped' when they are induced or persuaded by law enforcement officers or their agents to commit a crime that they had
no previous intent to commit; and the (USA) law as a matter of policy forbids conviction in such a case.
However, there is no entrapment (in USA law) when a person is ready and willing to break the law and the police merely provide what appears to be a favorable opportunity for the person to commit the crime. For example, it is not entrapment for a policeman to pretend to be someone else and to offer, either directly or through an informer or other decoy, to engage in an unlawful transaction with the person. So, a person would not be a victim of entrapment if the person was ready, willing and able to commit the crime charged in the indictment whenever opportunity was afforded, and that policeman or their agents did no more than offer an opportunity.
In the current case, using a non-police informant to make a buy from a dealer with police money would not be entrapping the dealer. They were only providing an opportunity to sell drugs. On the other hand, if they set up the deal undercover somehow (i.e., asking a small-time dealer for more than they can get ready access to, and then supplying money and busting them for a larger amount) and then busted the informant for trying to buy the heroin as well as the dealer, then that would be a case of entrapment.
I'm not sure it would make any difference though in Aus, in the end, because if they have enough evidence to make the case stick, then the court will probably not care under what circumstances you committed the crime. You would have to prove that you were coerced into committing a crime that you had NO intention of committing or reason to do so.
BigTrancer