The Collins was scoped initially under a conservative Aus government - including decision to go with conventional propulsion. After a change to a progressive government the Swedish (Kockums) design was selected ahead of the German offer - though not on the basis of the designer-nation's politics (after all, the same era saw the ANZAC frigates built to the German MEKO-200 design). If politics was the driver, why did they select the German design for surface combatant (vs British Type 23, Dutch M-class, Canadian Halifax options)?
The present change in propulsion (and design) is the evident element of strategic shift. Look at comments since that moment, made by the Aus Defence Minister, about forecast US-basing arrangements in Aust. It's likely the nuclear submarine propulsion was a carrot offered to Australia to sweeten a basing deal. Also, a functional consequence will see the RAN operate the Collins for a decade longer than the French project would have meant - which is an interesting challenge, as if the evolving strategic picture makes the Attack class unsuitable how is a smaller, older sub going to be a good interim platform?
To conclude, the Aus government has previously selected conventional propulsion as it permits local Aust construction and operational sustainment - a priority to make high expenditure palatable (defence expenditure = many long-term jobs). This new project effectively precludes local industry involvement in construction and maintenance of propulsion and combat system elements, unless Australian industry is considerably and rapidly developed in the next decade before construction commences. As a result, this complicated situation offers great opportunities to many people, but considerable challenges as well. It is genuinely interesting to observe.