Hello @jbasford, welcome to the forum!
If I understand it correctly: you already have got a site-to-site VPN between net 192.168.10 (NZ) and 192.168.20 (AU) up and running. Now you want to relay ALL traffic from NZ through the AU router?
If so, perhaps something similar to this is applicable: “Site-To-Site VPN tunnel while accessing internet from one router”
EDIT:
Assuming you are using IPsec, you may skip the “road warrior” part at the end of the post as I assume that NZ is already configured as an IPsec initiator due to lack of public IP.