Apple Watch Ultra 3 (NL) cellular in Australia?

Your provider has to support the Apple Watch. Have a look at the related article, listing all supported providers for the Apple Watch models.

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To cover the LTE and UMTS bands used around the world, there are two separate cellular models for Apple Watch. They have been optimized for use in the country of purchase, and not all models work in all countries or regions. There is no single model that supports cellular service worldwide. To see which carriers in your country or region offer cellular service for Apple Watch, refer to the chart below.

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copied from: Apple Watch - Carriers - Apple

Do you want to use it with your NL subscription (roam) or are you looking to add an Australian eSim to the Apple Watch Ultra 3?

The first I would not do (roaming), the second option depends on what your AUS provider offers. Mostly it is an add-on to your airtime/data plan of your phone.