Hopefully someone can 'Explain like I'm 5' to me :)

I'm moving to Melbourne for work in January and as this is a long term move I will be getting a regular Australian mobile number for day to day, but I also need to keep my NZ number for 2FA (& for contacts and for when I come back to NZ), I have an iPhone 12 mini currently on a 2degrees monthly plan but have read to change it to a prepay before I leave.

My question is - what is the best/easiest way to do this? 
Have 2 separate phones 1 for NZ and 1 for Oz (this seems silly but...)
Dual sim - 1 NZ sim and an esim for Oz... (don't know much about esim)
Any other way?

Basically I need to have an australian number to use for work, and signing up to services etc but only need the NZ number for receiving 2FA texts (mainly my bank as will be keeping it open and sending money over) 

Many thanks!!