Rovic's DH here. A QEII Covenant is the usual method (and there are lots of them).
Another option is a Land Covenant where whoever owns the hillside has to plant and maintain the hillside for the benefit of the rest of the subdivision land (and any other adjoining land that wants the benefit).
Kakapo's suggestion of calling your local QEII rep is best as they can advise you what the best option is to suit your needs.
Consider the likelihood of how long you want the hill protected. If you think there's a possibility that you'll want to use it for something else further down the track find out the process for removing a QEII covenant.
Good luck 