Hi Phoenix
OK - so your father will be "aged" when he turns 65, your mother some time later, so as long as he is 65, or over, on the day they apply, they can make a valid application for a contributory parent visa, while in Australia, giving them a bridging visa to keep them lawfully there, after their visitor visa expires. Again buttuming there is no condition attached to their visitor visa that would prevent an application.
Again, there is a danger that they could be considered as non genuine visitors on entry if, for whatever reason, they were questioned and it became apparent that they intend to apply for permanent residence in Australia and have sold their home in the UK. So this would be my only reservation.
So to summarise:
as long as they get let into Australia on a visitor visa that has no condition preventing on-shore applications, and that provided your father turns 65 before the 3 month stay authorised by the visitor visa expires, you can apply for an on-shore contributory parent visa on-shore, after your father turns 65 and before the expiry of the 3 month stay.
Tony
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