The only way to get off PPT to the south is via TN or NZ to AKL, neither of which can you fly as part of a GE or OWE fare.

You are better off traveling to TBU from AKL then SYD as the flights from SYD connect through AKL. Again, you can't do this on oneworld as the only carriers flying the route are NZ and WR (Royal Tongan).

Your oneworld *ONE4 ticket would look something like LHR-MAD-SCL-PUQ-MPN-SCL-PPT//AKL-BNE-CNS-SYD-JNB-NBO-LHR. You would need to ticket PPT-AKL-TBU-AKL separately. If you originate from CAI rather than LHR you could save £500-3000, depending on the class you plan to fly.