That's really interesting, Moksha.
I wonder whether it might be that the pollinator of the white/yellow one is simply not present in your district?
Do you know which part of Australia it is actually native to?
could it be that it needs a very specific pollinator, and can only set seed if it is grown in an area where this pollinator also lives?
(Perhaps hand-pollinating the flowers would produce some seeds for you?)
Trish

Well they're both in my garden, white in front and purple out the back. Ants love the flowers on both plants - before, during and after flowering, so I think they're the pollinators of the purple.

I was wondering if it might have something to do with hybridization. I think the purple is the natural colour while the white and pink are man-made so to speak. I might be wrong there though.