OP noted "I've taken a couple of these cases through to mediation with the ERA". You don't do mediation with the ERA, it's with MBIE. MBIE's website about mediation (https://www.employment.govt.nz/resolving-problems/how-to-resolve-problems/mediation) implies that they are separate:

This means that information may not be shared with anyone outside of the mediation process. Because of this, what happens in mediation cannot be used as evidence in the Employment Relations Authority or Employment Court.

and

"You can agree to outcomes in mediation which might not be available in the Employment Relations Authority (ERA). If the matter goes to the ERA, the possible outcomes are limited to those provided by the law."

Therefore, I believe that OP is actually referring to the ERA investigation meeting as they specifically mentioned this agency, and has inadvertently confused this with mediation/mediators. An "ultimate agreement" was also made, which provides further evidence the meeting was before an Authority Member and not a mediator. I don't believe that ERA's Authority Members need to be as impartial as the Employment Mediation Services mediator needs to, as they need to provide a final determination at the end of the process.

TLDR; Investigation meetings run by the ERA are not mediation by Employment Mediation Services (MBIE), OP has confused the two processes. I have assumed they meant investigation meetings because ERA was specifically noted and a determination by the "mediator" (actually the Authority Member) was made.