There's a tread about this ATM on Pirate 4X4.
It appears the worst side effect from "overcooling" is that water might condense out of the fluid and freeze in the auto.
Not a problem me thinks in Australia, but it was reported as an issue for guys in the northern US/Canada in winter (night time temps of -40?C

) Even then, there was no damage, only funny shifting until it warmed up.
Apparently though, autos are "optimised" to run at about 75?C
I'd suggest in Australia it would bad hard to cool an auto well enough that it ran much below that most of the time.
It might drop below that after a long down hill, but it wouldn't be for long.
Steve.