Here's some info on the jacksons bay haast launching facilities.

Honesty box for $10 ramp fee's
Best times to launch is any time except 2 hours either side of low.
Initial concrete ramp onto the beach with small (tennis ball and bigger) stones in the sand. Doesn't look much but its OK. Can have a wee swell so timing your approach to drive on boat to trailer is important.
4wd required and diff lock if you have it for a heavier boat.
Not good in a swell from the north or northeast. All else is not too bad.
Southwester wind can come up quickly and watch for southwester arch in sky as clues.
parking generally not an issue.
Small commerical fleet operates out of harbour.

Only protected boat launching location for 100 odd KM's
Can motor over to wharf to load vehicle person

If going toward Jacksons head watch out for a very large rock called Hapuka rock. In breaks in most conditions but is right in passage around the head (if heading south) that taking a wide berth of the head itself. Could be very easy to hit if your not careful.

Recommended 2 boat minimum as remoteness and lack of immediate coast guard means lack of reliance from outside help. Aux outboard is a good idea.