ABC Radio Today, page-18

There's also exports.

Perdaman says they're going to export most of their production.
Which makes sense; their location is about the same distance to SE Asia as it is to the rest of Australia.
Also they have connections with India, so I'd imagine a fair amount will go there.

I did some rough calculation, and Aus can sustain all three plants (including Incitec) once exports are taken into account.

Wouldn't be surprised if Perdaman/STX ends up splitting Aus in half between them.
Perdaman/Incitec would sell to NT/QLD/NSW
STX would sell to WA/SA/VIC

Traders and users would prefer one plant (depending on their location) - however would have contracts with both companies to assure supply.