I tend to spot something, wonder what on earth it is, photograph it, then come home to use trusty ol' Google to find out. Or phone/email somebody.
In this case, I Googled.
There's a couple of these plaques on the bridge which is an extension of Memorial Drive in New Lynn, and crosses over the railway trench there. HN-HO-72 is the accepted traffic loading design specification as per Transit New Zealand's Bridge Manual. HN is normal load, HO is overload. There's a .pdf here: for those of you who are engineering-minded, check out pages 3 to 4 especially.
My curiosity is now satisfied for another day.