Australia insisted on diesel subs in the first place, and for whatever reason they chose the French proposal over German and Japanese subs that were designed around diesel from the start.
Australia wanted a gas sub that got the performance of a ssn. Was never going to happen and geopolitical reality finally caught up with them. There were groups in Australia including Tony Abbott that wanted nuclear from day one.
That said, not telling France was a dick move but what can we expect from the resident.