If you're doing it via the TREIS REST interface you can see the data schema at
https://infoconnect1.highwayinfo.govt.nz/ic/jbi/TREIS/SOAP/FeedService/main.wsdl (via your browser)
e.g. using curl:
(I use the insecure option to save having to store/validate certs)
A request might look like this:
Accept-Encoding: gzip,deflate
username: YOURUSERNAME
password: YOURPASSWORD
Connection: Keep-Alive
User-Agent: EdgeFrontier/6.2
If you're using SOAP you need a WSSE header added with your API username and password. The wsdl doesn't explicitly ask for it.
eg:
<soapenv:Header>
<wsse:UsernameToken>
<wsse:Username>YOURUSERNAME</wsse:Username>
<wsse:Password Type="PasswordText">YOURPASSWORD</wsse:Password>
</wsse:UsernameToken>
</wsse:Security>
</soapenv:Header>
<soapenv:Body>
<tre:GetTreisInfoRequest/>
</soapenv:Body>
</soapenv:Envelope>
I hope these get you started.