Is Belong compatible with Cellular Apple Watch’s?

The following is a list of carriers that support the Apple Watch

https://www.apple.com/watch/cellular/

Belong is not listed in the above list.

If you do not want to switch carriers, you can get a GPS only Apple Watch (less expensive) and do Everything a cellular model can do As Long As your iPhone is nearby

No. They just buy cellular services from Telstra and do not support Apple Watch. If they are your carrier, you can purchase a GPS only model, which will work as long as your iPhone is always near your watch.