At time of placement there is road access in the weekends only, from the Fabians Valley turnoff and up Bartletts creek. This is subject to change at any time due to logging operations. check out http://www.doc.govt.nz/parks-and-recreation/places-to-go/marlborough/places/mount-richmond-forest-park/park-access/ to see if the road is open. At times this road can be driven as a loop, coming out at the Onamalutu domain via Kiwi road - currently mountainbike only on that side.
If you have a good off road 4x4 you will be able to drive to the cache. Town cars should make it to the intersection a couple of hundred meters down the creek if driven with care. Take care during rain as the fords can rise fast.
No cellphone reception, so if in doubt give it a miss.