Why not, like the Singaporean and Indonesian armies, get the 2nd hand Leo2A4 from the German stock and upgrade them to the latest standards.
With all this talk of 'new' MBT's for Australia... I have to seriously ask, what would be the point?
I do agree that more MBT's might be appropriate if units which either operate armour, or need to operate/train alongside armour are going to be distributed to more posts around Australia. The same would apply if it was deemed advisable for Australia to have a larger pool of MBT's to draw upon.
What does not make any sense to me, given how recently Australia got the M1A1 AIM Abrams tanks, would be for Australia to dump those tanks and go with another roughly equivalent design.
IMO it would make far more sense to either get more M1A1 AIM tanks, or if more and improved versions were desired, have the existing stocks upgraded to an appropriate version along with ordering the extra numbers required. If it was deemed not practical to upgrade the existing versions in Australia to the desired standard, then order the version from the US and trade the existing M1A1 AIM tanks in for re-manufacture.
That seems far more sensible than needing to train crews and support on how to operate and maintain a different MBT with an entirely different support chain. I do not see a significant capability advantage other modern MBT designs have over various versions of the Abrams to make such a change worthwhile. If there were a generational difference in capability, like there had been in switching from Australia's Leopard I's to the M1A1 AIM Abrams, then the situation would be different.