Some areas are part of Kura Tawhiti Conservation Reserve and managed by the DOC, others are private property.
For access to the main Castle Hill area, you can park in the lot available for the Castle Hill Park which contains some walking trails. These walking trails will also take you to Quantum Field or Spittle Hill. This is one of the easiest areas to get up and get going unless you want to fit in a good hike.
If you want access to Flock Hill, Dry Valley or Cave Stream, take a 10 minute drive further north along 73 to park at the Cave Stream Park area. There will be an obvious park sign. You will then need to take a bit of a hike to get where you need to go.