Given that there is almost zero possibility of the ADF engaging in combat on Australian soil and that there is a much greater chance of it participating in some future UN or Allied operation in the Asia Pacific region or elsewhere, I would imagine that consideration will be given on operating conditions/terrain likely to be found in these countries. It will be 'inconveniant' if a vehicle is suited for ops within Australia itself but it too heavy for many of the bridges found in South East Asia or is too wide or has too high a profile to effectively operate on many of the narrow dirt tracks found there.
I find this a rather curious argument. Not having a go at you in particular but rather at proponents of the idea, that we have to base our decisions on the major combat assets we choose to acquire for our Army, on the idea that we have to choose some theoretical design featuring some generic specifications, that will thus fit on some theoretical jungle track or some hypothesised 'bridge' in the wider Asia Pacific region.
Every major power in our region operates heavy armour. To employ this capability, even if we are somehow enemies, they will need to cross the very same bridges and traverse the very same tracks, yet they choose to employ heavy armour within their respective force structures, whereas it is supposedly a terrible idea for us to do the same thing...
For some obscure reason and despite the reality of the last time we engaged in jungle warfare within this region (successfully employing heavy armour in the process) it has somehow become fashionable to assume that we would be unable to do so again...
Out of interest our biggest, heaviest armoured combat vehicle is the M1A1 AIM tank, roughly 63 tons, 9.7m long and 3.6m wide.
Indonesia's Leopard 2's are about 63 tons, 9.9m long and about 3.7m wide.
Malaysia's PT-91M's in upgraded form are about 55 tons, 9.6m long and about 3.6m wide.
The T-90 tanks Vietnam is reportedly buying are about 55 tons, 9.6m long and 3.7m wide.
China's most modern reported tank is the Type 99A2 at 58 tons, 11m long and 3.5m wide.
Thailand's most modern tank is the T-80M Oplot, about 51 tons, 9.7m in length and 3.4m wide.
Those are the realities of the heavy armour currently being or intended to be operated within our region. Rather interesting it is only our heavy armour that is being called into question...