Does your phone have a physical SIM tray? (iPhone 15 variants that Apple designed for the U.S. market do not.)
Is your phone carrier-locked? If it is, you will need to convince your carrier to unlock it before you can use a SIM from any other carrier, whether that is a physical SIM or another eSIM.
How to unlock your iPhone for use with a different carrier - Apple Support
Is your plan to transfer your phone number to the Australian carrier, and perhaps also to stop subscribing to the Swedish phone service while you are in Australia? That would definitely require help from the phone companies. I'm not an expert, but it also might mean that when you came back from Australia and restarted service with the same phone number, the Swedish carrier would have to issue you a new eSIM.
Or is it just to add an Australian SIM with a different phone number than your main (Swedish) one – making your Australian SIM the default one for outgoing cellular phone calls, and for cellular data?
As you can see, this may quickly get into areas where you need the carrier's (phone company's) assistance.
Use Dual SIM on iPhone - Apple Support