The ATSB would do well to focus on why the Dash 8 crew missed the presence of the C172 in the first place. Look at SOP. Do they call on CTAF just so they can commence taxying, then go off CTAF frequency to Centre for IFR traffic and to initiate SAR? Maybe their second radio is still tuned to CTAF, but is one pilot exclusively monitoring it to the exclusion of all other duties while the first guy is on Centre? Highly unlikely - too much would be happening with the checklist for that to work in real life.
Not saying this is foolproof, but our operation calls Centre first, then switches to CTAF until airborne. After that initial contact, Centre frequency is then set to a very low volume so it does not interfere with anything that could pop up on CTAF. If Centre want to talk before we are clear of the circuit area they get the old "stand by" reply.
Also does their SOP involve much 'quacking' to the instruments and running long checklists while taxying? The distraction of this can drown out some lone voice making a cryptic transmission like "XYZ final runway..."
Made worse if said transmission does not repeat the place name at least twice.
As for the unfortunate 172 pilot, if CASA get nasty his wallet and licence could cop a flogging. If he was legally operating after last light, that would not be an issue. If he was not, well he's stuffed. But assuming he was legal, much may depend on whether his admission to sighting the Dash 8 was early on final or very late on final. If early, he is probably due some attitude readjustment, courtesy a friendly CASA FOI, who will not be happy until the pilot is not happy.
If he saw the Dash 8 late, he could argue that it was better to land because he had plenty of stopping distance. Whereas, a go around with potential engine malfunction could have risked a collision with the other aircraft. Anyway, that would be my story.....
If this had occurred in the USA it is unlikely to have even raised eyebrows. 'Landing after' is routine. Probably less risky than going around in busy airspace. This, in the land where they really did invent aviation. For us to re-invent.