Read the award rules and it becomes obvious. However the awards are priced differently, the OW award is 35K in Y and 75K in J (not 20K as you seem to be assuming). I'm sorry to keep repeating this, but the aa website is quite clear about the award cost and rules, you simply have to spend 2 minutes to read all of the rules ... I can't type 2 pages of rules and tables into an FT posting.

One key difference is in the number of stopovers allowed (OW awards are mileage based and allow a large number of stopovers, at the expense of flexibility as per the rules, thus well suited to "circle" type of trips or other convoluted routings; the regular AA awards are intended to be for point-to-point round trip but have some provision for stopover and/or open-jaw but with limitations). For your example SYD-AKL-SYD is better done with the regular AA award while something like SYD-AKL-CHC-MEL-SYD would suit a Oneworld award. But getting that LA flight booked is very hard (only 4 services per week, or 20 F seats per week for sale, maybe 1 or 2 available for awards and those are taken for SYD-AKL-SCL and not often available for SYD-AKL. In practice Oneworld awards are irrlevant for Australia/NZ travel, you have to also go to HKG/BKK/SIN etc.