Apple Watch SE gps + cellular sim help

The cellular watch must be with the same network provider as your phone.

The cellular watch uses your phones number via an Esim built into the watch.

And as far as I know Telstra doesn't support the cellular watch.

Apple Watch - Carriers

Reading your reply brings me to another question.... does my cellular service need to be contacted if I pair an Apple Watch with my phone? are there any extra charges associated to using a watch that is paired with mysiphone?

If you choose to activate the Apple Watch cellular, then the iPhone cellular carrier, if the support the Apple Watch, will change a monthly fee. In the U.S. the fee is around $10 per month. Similar fees exist in other countries. Check with your carrier to get an exact price.

if you choose NOT to activate the Apple Watch cellular, there is no fee and the Apple Watch behaves as if it were a GPS ONLY Apple Watch.

The only extra charge is a monthly fee for a cellular watch,if you just have the GPS only watch there are no extra charges,this explains it;