My transponder arrived today so that was a good excuse to take a break from the baffle installation and finish off the panel instead. I decided to go with the Trig TT21 transponder.
Even though technically you can still fly with a mode C transponder in Australia till 2017, as of February 2014 all new aircraft to the register in Australia require to have an ADSB-out capable mode S transponder. Not all mode S transponders are ADSB-out capable. Watch it, the shops selling them won't tell you this, nor will they tell you about the Feb-2014 limit for new installations as they're trying to get rid of their now obsolete stocks. I nearly got caught with this.
Anyway, the Trig TT21 & TT22 are one of the few 2.25" ADSB-out capable transponders out there for the moment so there was not much choice.
Installation was straight forward as I had already done all the wiring etc. just a matter of installing the panel unit and main unit at the foreseen spots, hook up the wiring and that was it. The panel is now finally complete.