I've done it, in the days before I got a gps. If you have a week to spare then I recommend doing the loop Middlemarch- Rocklands Station -Paerau- Poolburn Reservoir- Alexandra - Ranfurly - Middlemarch. The first part to Alexandra follows the Old Dunstan Trail. This leg can be done in 2 days on a bike, but you need to be equipped to camp out one night. 2 mighty big hills to climb/push but 4 glorious downhills. This leg needs careful planning and can only be done in the summer as the Dunstan Trail is closed over winter due snow on the tops-the two major mountain ranges you cross are very exposed with no shelter anywhere. The Rail Trail (Alexandra - Ranfurly - Middlemarch) can be done in 2 big days. Can be a bit boring, the gravel surface is not hard packed in places and can be slow going. Need fat MTB tyres.
My mate and I traveled light and we utilised a 4x4 support vehicle driven ahead by a novice 4x4 driver -who, whilst on the 4WD/MTB Dunstan Trail, had instructions to stop whenever she came across possible difficulties and at pre-arranged rendezvous points. She also opened all the gates for us so we sailed straight past whilst she then closed them.
Brilliant! The vehicle was very much appreciated for avoiding what would have been a 2+ hour, 8km push, getting steeper all the way, up to the top of the Rough Ridge range i.e. from S45 21.018 E169 53.963 to S45 20.733 E169 48.370.
There a few caches quite close to the Middlemarch-Old Dunstan Trail route: GC153, GCKYMN, and the rather neat GCNB1V (very close to where you turn onto the Linburn Runs Road) and GCTN0G.