Have you taken a look at the HASANZ Register for an Occupational Health Nurse?

Yep, got nowhere fast ...

I've come across this issue previously, where I couldn't find anyone who could help with mercury biological monitoring (no one could do the air monitoring either). I ended up finding a General Practitioner with a special interest in occupational health (GPSI). I explained what they were potentially being exposed to, and they sorted out the blood work and interpretation.
In your case mild steel and low alloy steels, the recommended biological monitoring is for chromium, nickel and manganese (sourced from Breathe Freely NZ NZOHS)).

You need to engage a registered Occupational Hygienist. They are in short demand in NZ suggest you contact the New Zealand Occupational Hygiene Society

Hi Leah,
I would recommend you contact a Occ Hygiene Specialist to provide info on the quantification of the fume and then engage an OH Nurse for your testing and monitoring of staff. Feel free to email me (rmcaulayatnorthtecdotacdotnz) for specific recommendations around who we have used.
Regards
Rob

Safe and Sound advertise Biological Exposure Monitoring services within their brochure.

I'm not sure how far outside of Christchurch they operate though.

Leah as you are in Auckland, I'd recommend Suzanne Broadbent at Haz Expertise. She is experienced in biological monitoring and testing and I've worked alongside her when testing Fuel tanker drivers for inhalation fumes. Being an Occupational Health practioner and a hazardous substances expert, I'm sure she could help.

Yep, I would go for Suzanne as well.

Awesome, thanks team.

We are using LM Safety for the workplace monitoring, but they should also be able to help you with the health testing, either that we get advice from an Occupational Physician when needed, they have a website as well. https://anzsom.org.nz/