Firstly, in Queensland the police don't need a warrant to search anything, they only need "reasonable suspicion". Check out
this link for an explanation of Queensland's extreme search laws.
Queensland police can and do go through sharps bins, including emptying them out on the back seats of cars etc. There's no need for them to send it anywhere, unless there's some specific testing being done. I've even heard of someone having hundreds of syringes emptied out, and being left to put them all back into their bins.
Of course ethically the police shouldn't be doing this, because it reduces safe disposal practices. People just dump their fits wherever if police routinely search through sharps bins. I saw the return rate drop to below 30% in a needle exchange in one area where this happened.
If they find drugs inside your sharps bin - including in some cases empty baggies with residue, you can be charged and that can be used in court. I've heard heaps of stories of police threatening to charge people for the residue left in the end of the needle too, but I imagine (or hope) a magistrate would laugh that out of court.
Having said that, I've also heard a few times of police that find used sharps stored in a sharps bin, giving a pat on the back for being responsible.