How to separate out NZ state highways using type attribute in roads layer?

After posting this I realised I had forgotten to mention that I am working off downloaded NZ road shapefile from http://download.geofabrik.de/osm/australia-oceania/ - so perhaps this download does not include all road attributes. I also noted a New Zealand discussion on highway attribution - http://wiki.openstreetmap.org/wiki/Talk:WikiProject_New_Zealand so I will follow up with the New Zealand OSM group.

In New Zealand there is a clear distinction between roads that are state highways and roads that are not. They are managed and signposted separately. Although open street map data has a variety of entries in the 'name', 'ref' and 'type' fields, it is not possible to create a valid query just for state highways. This is primarily a problem of consistency in attribute usage.

I would like a simple query to be able to select all New Zealand state highways and think that many users of the data would find this useful. I am aware that different countries have very different hierarchies for roads and different administrative and management classification systems.

My simple suggestion would be to add a new type to the roads layer = 'NZ State Highway', and apply this to all state highway road sections in New Zealand. Is this a reasonable approach? How could this be done better? How could this be made more generic to be useful to the wider community? Should another attribute be added to roads? Should the new type just be 'Highway' - even though this means different things in different countries?

Examples of state highway attribute data existing currently.
1. where 'name' like "SH" "State Highway" "SH36" "Highway"
2. where 'ref' like "SH" - in practise either "SH2" or just "2" has been used
3. where 'type" in ('motorway', 'primary','trunk') - although this usage is not consistent - some state highways are classified as 'secondary' and not all 'primary' roads are state highways