The most cost effective (yet still kinda pricey) way to get from the North Island to the South Island of New Zealand is the ferry from Wellington to Picton. Since the only way to really get around the country is by car or tour bus, we brought our rental car along on the journey. The trip is about 50 nautical miles that take about 3 hours to travel. The views were stunning and moody.
pro tip: grab a flagon to enjoy once you’re on the ferry from one of Wellington’s many breweries (a flagon is the NZ equivalent of a growler)