Flying from AKL / SYD to SCL then onward to PPT would indeed be backtracking. You could fly to IPC then pay to fly back to PPT or use miles.

Indeed, looking at ITA IPC-PPT is either £320 or £850 return in Y, or J respectively which is cheaper than the £1000 one-way flights with a layover that it gives for SYD / AKL-PPT.

Not sure if there is a bug with ITA as the AKL-PPT flights seem to go via SYD and the SYD-PPT flights via AKL.

Do bear in mind that open-jaws / land segments count against your total number of segments. It maybe cheaper to book a return anyway and if it saves a segment then so its a double bonus.