Picton is a city on New Zealand's South Island. If you're arriving on the ferry from Wellington it's probably best to ask people on the ferry for a ride. Standing with a sign at the stairs when drivers are going back to their cars works well.
It's faster to first head South to Blenheim, apparently most traffic to Nelson takes this route.
If you decide to take the Queen Charlotte drive it could be best to wait at the roundabout near the ferry.