All the main State Highways in New Zealand are measured out with posts at kilometre intervals. Kposts. They look like this.

To the traveller, they can be useful in a number of ways when you know the code.  They'll tell you how far you are from a given point (like the next petrol station when you are real low on fuel).  They are handy if you are looking for a particular attraction or place - in fact they can be more use thasn using the odometer. They can provide an answer to the eternal question from small persons in the back seat:

So what is the code?

Transit New Zealand, which builds and maintains our roads, has divided each highway into sections, called reference stations,  about 16km apart - but there's no set distance. Together they have been given the grand title of the Location Referencing Management System.  For simplicity I refer to them as sections.  

The first section (or reference station) on any highway is the zero section because the road starts at a kilometre distance of 0.0 - obviously. Travelling along that road from its start point, the first kpost you would come to would be the 0 / 1 kpost.  That means it is 1km along the zero section.  The next would be 0 / 2 and so on.  At some point  a new section starts. On SH 2 for instance the second section starts at the 18km mark.  At that point the kpost would read 18 / 0. The one before it is 0 / 17. The one after is18 / 1, and the next 18 / 2. and so on. 

The mathematically minded reader will have already worked out that one can therefore quickly calculate the distance from the start of the highway by adding the two figures together.  Thus if you arrive at the 24/6 kpost on a highway you know you are 30km  (24 + 6) from the start of the road. 

There are a few other clues that will help you locate yourself - in the absence of a portable GPS unit.  Bridges are all numbered and the numbers are related to their position on the particular highway. A typical bridge sign looks like this:

Thus a bridge numbered 2543 would be 25.43km from the start point of that highway. Bridge 98734 (the Hawkstone St overbridge on the Wellington Motorway) is 987.34km from the start point of SH1.

You'll also see ERP signs.  Like this:

ERP = established road position.  It is an accurately measured point along the road.  Which raises another point.  The kpost system is somewhat less than perfect.  If you check the kposts against an accurate odometer there will be a difference.  The reasons are many - and are largely irrelevant for our purposes.  To start with, how many car odometers are actually that accurate? They are frequently out by up to 5%. And you don't need to be super-accurate anyway - it's a holiday not a navigation training exercise.

In my road notes I show the kpost references like this:   24 /  6  In this particular case, the reference point for Thames on the Coromandel peninsular, the town is 6kms along the 24km  section of SH25. 

The next two figures I show, which look like this:  32.2 / 197.7 are the distances in kilometres from either end of the particular section of road covered in that section of the guide. Again, in the case of Thames, it is 32.2 km from the start point at the northern end of SH25, and 197.7km is you started at Waihi, the southern end of the highway.

For the official explanation go to this Transit New Zealand site.