This subject seems to pop up every 6 months or so.
The Convair is a good machine, but we have too few airports equipped to handle it.
The RAAF tried a MATTS system in the back of a Herc in the early 80's. Unfortunately their lead time to fight a fire meant that the fire usually got the upper hand. Not enough flexibility.
The DC-10 was a knee-jerk, and unsuitable for the job here.
Australia already has the answer; lots of Airtractor 802's and very skilled and knowledgeable crews, but it seems the authorities here want an overseas solution. Can't imagine why, as this usually results in a very expensive, less than perfect outcome.