The short answer is, not much. Commuter trains stop at Waikanae, except for the Capital Connection which heads through Waikanae northbound in the evening, and returns the following morning — and even that train is under threat.
When it comes to buses there isn’t much either. The timetable for route 290 north from Waikanae to Otaki is given above: two in the morning and two in the late afternoon — with the same number and times of day back.
If you want to go further north then the option is a long distance bus. The timetable for InterCity to Palmerston North is shown below: three buses in the morning (and for some strange reason three different transit times!). Likewise there are three services in the afternoon. The fare is about $20 each way — and you need to book.
All up then, a quite unsatisfactory situation. The answer? See the earlier post on a proposal for a connecting diesel-multiple unit train between Waikanae and Palmerston North.