For anyone who is looking for Aeotec Doorbell 6 with AU/NZ frequency, you can buy it from Active Automation NZ.
After checking with different retailers in Aussie and NZ, the only place I found the Doorbell 6 was at Active Automation. In fact, Aeotec themselves don't have the AU/NZ frequecy Doorbell 6 listed on their site. So I was bit surprised to see it in stock with a third-party retailer.
Anyways, I ordered one and they shipped it out the same day. Can confirm it works perfectly fine with the AU/NZ z-wave. Have added it to the hub. Just have to figure out some automation rules to trigger mobile notifications. I'll do that over the weekend.
Had a good experience with them, so thought of sharing it with my fellow AU/NZ hubitat owners who are looking for Aeotec products.