Can't add bag on codeshare domestic QANTAS fight.
 

Booked Seat Only through AirNZ website. AKL-SYD and then Qantas codeshare to Adelaide

Find I need to take a checked bag now. Can't do it online and call centre also say it can't be done. I can buy a bag for the ANZ leg to SYD but not the Australian domestic leg.
Given that Qantas don't do "no checked bag" fares domestically it all seems a bit inconsistent. Looking at $170 for excess bag at the airport. I've put in a OneUp. :-)
In June, "Air New Zealand Chief Executive Officer Christopher Luxon said the codeshare will see two of the world's best airlines offer their customers a full service experience when they make domestic connections within New Zealand or Australia."
Not feeling the experience.

I made enquiries previously about this with NZ. Got referred to the baggage page. Not helpful. I'd call Qantas (get the PNR from NZ 1st?) and confirm. Hate the idea of getting hit with fees at the airport.

I basically decided to fly QF/VA through instead. Couldn't be bothered with the hassle. 0800 808 767.

Your question needs more context. I assume you purchased a seat only TT S2S fare but want a bag on the QF domestic flight?

Which will happen as all QF tickets include checked baggage. At SYD go to the T1 QF check in area and drop the bag.
OP is trying to solve a problem that does not exist with QF.

Airport info SYDNEY (SYD) - amenities, transfers, transport Click to open the wiki

No want bag all the way from AKL-ADL. I omitted to say upgrade to seat+bag was unavailable on website which sent me down the 'add a bag' rabbit hole.

Problem now solved after second phone call. Thanks all.

You will be doing that anyway.
As SYD is your port on entry you must clear Au immigration, customs & quarantine with all your checked & carry-on luggage. Then take the bag to check-in/bag drop for your Au domsetic flight.

Baggage does not fall to operating carrier, actually. There are IATA rules known as "most significant carrier" rules that apply instead. Technically, as the whole thing is on one itinerary ticketed by NZ, the NZ baggage rules apply across the whole thing as they are the most significant carrier (being the only international sector) and not QF. In theory, QF do not have to offer you a bag at all.

Whether they do or not, is a different question.

MSC only applies to a single checked baggage "journey". In this case the OP has the option to break the journey in SYD because they have to uplift their checked bag in SYD and drop it off again with QF. While it could be a single journey for MSC purposes - eg if the first operating carrier tagged the bag all the way through to the final destination such that the manual intervention of picking up and dropping the bag off at the transit point did not constitute a break in the journey, because the tag is all the way through - it is also capable of being broken into two journeys in which case the rules of the operating carrier would apply to each "journey".

NZ and QF have an interline agreement, and all news reports indicate that the codeshare agreement has "easy baggage transfer" which would indicate to me that if booked on a single PNR, you're getting a single bag tag - meaning MSC rules apply.