We went through similar fun and games when older daughter parked in the Wellington City New World on Wakefield Street for longer than the allowed 90 minutes. I took objection to their letters / emails to me (as the registered driver) as I had not parked at the site and certainly had not given anyone else permission to park there.
PESNZ are the R soles in the relationship. When they passed it on to Baycorp I was then able to put my argument to Baycorp, who are held to a higher standard of care given their privileged position as a debt collector. I was subsequently advised by PESNZ that the debt had been booked in error, i.e. Baycorp can't been happy with the legality of going after the debt.
I treated it as a bit of fun. Your case is a bit different though as you were there and so apparently subject to their rules. Arguably if you had already withdrawn access to your NZTS details then you can't have agreed to their Ts and Cs (assuming they're the same as at NW) as they include allowing access to NZTA info.
Good luck.