If I book SIN-PVG return, but show a PVG to HKG flight for transit, will this be ok?

So your flight route should be Singapore-Shanghai-Hong Kong-Singapore, right? If so, you are ok for 72 hours free transit.

Sorry I wasn't clear - book one ticket, sin-pvg return. Then on a seperate ticket show pvg to hkg one way but i iell leave on my original flight?

But you enjoy 15 day visa exemption to China if you hold Singaporean passport. So, nothing to be worried about about this transit.

Im an Australian passport holder

So, is your itinerary sin-pvg-hkg-pvg-sin?

It would be sin-pvg-hkg(or other city)-sin

sin-pvg-hkg(or other city)-sin? 72 hour visa free transit is for one stop only in one eligible city only. That is to say, you can not avail this if you will transit via two airports in mainland China. HK works as the third region, so it is ok for you to avail free transit permit.