I have come across a transcription of the birth certificate of one of the children on Ancestry - Joseph Henry Rochez - that can be downloaded here:
http://tinyurl.com/kxrp2em

Unfortunately this doesn't give much information. Do you think if my friend purchased the certificate for the other child born in South Australia, or the first child born in Western Australia that more information would be provided?

There seems now to be two points against Joseph Rochez/Joseph Packer being the convict John Roche:
1. The marriage to Mary Egan took place only a few months after the convict John Roche arrived in WA
2. The convict John Roche was still in WA when Rochez/Packer was in South Australia.

But if Packer is not the convict John Roche, then the question still remains as to who Joseph Rochez is and how he arrived in WA...

It certainly is very intriguing...
Thanks for all your help with this. The info on the Rochez connection was a fantastic find!