I got the code shares from web site AKL international departures https://www.aucklandairport.co.nz/flights
Have read Expert Flyer has this info.

After issuing you can change flights. Changing a flight on the same route has no theoretical cost. But some airlines charge a "service fee". To me that is not in the OW rules, but happens. So changing to/from a codeshare is possible to maximise ff earning. Some airline phone agents may not be keen to do, but if changing to their airline may be more amenable.

FF earnings with Oneworld are based on the marketing carrier (flight number) and an eligible operating carrier.

Edit
https://www.flightaware.com/live/flight/BAW4550