There is a published MPM, ie maximum permitted milege, value between any two pairs of cities which is used to determine what is an allowable routing. For SYD-BKK the MPM is 5611 (one way). Revenue fares usually allow you to exceed this by 25%, but for Aeroplan awards, only 5% over is allowed.

You can use a site like great circle mapper to calculate the mileage for various stopover points. If the mileage is within 5891 (5611 + 5%) then it should be allowed on a SYD-BKK reward ticket.

As an example, MPM for AKL-BKK is 7147, and the AKL-SYD-BKK routing comes to 6006 miles - so SYD is a valid stopover between these two cities.

Probably, the best way to get an AKL stopover is to book an "open-jaw" reward ticket with SYD-BKK-AKL routing, and separately purchase a cheap one-way AKL-SYD flight for the return.