Whatever the explanation is, it will also have to shine light on the divergence between change in value vs change in debt (Fig 2) in the period starting circa 2002 when the value line started racing above the debt line. Also in 2008 when the value line plunged below the debt line.

Picking those two years suggests that some relationship with the state of the international financial markets might exist (2002 being the start of a massive bull market, 2008 when the GFC was hitting in full force).

Would any clues be gained from examining NZ's capital account figures.