"In Australia, nbnTM maintains that the Telstra copper it's taking on for FTTN is up to scratch all the way to gigabit G.fast services"
Except that G.fast only delivers 1Gbps over less than 100 metres of copper. The FTTN lines are several hundreds of metres in Australia. With FTTN already built, there will need to be many more nodes (mini-nodes) going into many (not yet built) small pits. At what cost? How long will this take? We can envisage a similar cost and time to build to the current rollout.
All to avoid FTTP, which is more than viable for most suburbs within cities and towns, as other countries are demonstrating on the ground right now. Where it's not viable in the boondocks, *then* there are several technologies to use.
No, the current Government in Australia wants to leave all of the above for the buyer/s of NBN. Expect a sell-off before G.fast, which means G.fast will likely not go ahead with the global transition to full fibre underway.