Jon Johansson commenting on Sunday's Q+A program pointed to the internal differences within National (to a lesser extent reflected in Labour) regarding the Constitutional review. I think the differences are somewhat deeper than that.
The "21st century" wing of National grew up with Roger Douglas and Ruth Richardson as ministers of Finance, the "Jurassic" wing grew up with Muldoon. The Jurassic wing remembers when New Zealand's economy was highly protected by both trade walls, but also by guaranteed access to the country formerly known as the Motherland. They grew up in a time when our prosperity was maintained by being Britain's farm in the South Pacific; when up to two-thirds of New Zealand's exports went to the UK.