As expected, when measured today, the mechanic confirmed that the rotors required replacement. They are the 340mm rotors.

I have ordered new brakes, rotors and a wear sensor from clickautomotive.com.au (very focused on BMW parts)- they're Australian based, and were happy to ship to NZ (costing $160 AU for shipping), but couldn't order through their site as it only listed AU as the destination country. They issued an invoice separately and I paid via Stripe with credit card. Still came out cheaper than trying to source it locally. 

Lucas, the sales manager there, is very friendly and super prompt with responses. 

Options I considered:

- clickautomotive.com.au - BMW focused, ended up ordering from them.
- www.micksgarage.com - seems to be a reasonably priced (reputable) rotors, and VERY reasonably priced shipping ($30NZD from Ireland) - well rated in some forums I read.
- www.partzonline.co.nz - had some of the parts, but the online catalogue was a little iffy with details (listing rotors as pads, etc.) - seemed reasonably priced, and responded to emails promptly.
- https://europartsaustralia.com/ - Price wise didn't stack up when compared to other options. Ended up being as expensive, if not more than local, and that was without GST.

... plus some more... Placing these here in case others have a similar need later! 

I also checked out the other options that people responded with here.

The main limitations with overseas suppliers I found is 1. the shipping costs - some were over $400, and 2. the shipping delays.

The main limitations with local suppliers I found was needing to reach out via email, very minimal online catalogues, some inaccurate details, etc.