Rassie there are strict rules at IRB level. In summary if one has represented a National Team or a National Second Team (Australia or Australia A in our case), then that person can not represent another country ever.
Representing the nation at age group (Under 20's) does not have the same limitiations unless there is no second team from that nation.
NZ and Australia have been accused of cherry picking PI players and while this may have occurred to a degree in the past (80's and 90's) most of the PI's in the Wobs and the Darkness were not born in the Islands, and gained all their rugby skills development from NZ or Oz. They are brown faced Aussies and Kiwis. Conversely many of the PI teams at Senior and U20 level have numbers of NZ and Aus citizens of PI extraction playing for them.
We picked up Aussie Mike Harris when he was unwanted by NZ rugby and he eventually became a Wallaby. I suspect that the Lords of Darkness hardly cried foul play when he played for us agains them. It was a win win situation.
Look at the the SH players in ENG, IRE, WAL and SCO who are there because of grandparents or parents lineage. Not a lot of Saffers but there are a few Kiwis, Aus and PI's who have qualified by residency or dual citizenship.
I think it was Shane Howarth from NZ who played a handful of games for the Welsh before the Welsh Grandparent link was proven false.
I can't think of export from NZ, Aus, SAF, or PI who has gone on to set the world on fire in their second NH country after not making it in their own land. Most of the donating countries are generally ambivilant about losing those exports.
John Gallagher (a Pommie policeman who played at 15 for the Darkness in RWC 87 who subsequently returned home to play Loig), and perhaps Tomaci Cama Jnr in the 7's for the Darkness may be exceptions.
Clyde Rathbone perhaps - SAF U20 captain wasn't he?