Peltier et al. (2009) present levelling data from 1967, over the the main crater of White Island volcano. 
Deformation data “reveals that shallow pressure sources (200–600 m deep) extending up to the subsurface dominated the long-term deformation pattern consisting of inflation/deflation cycles”. The authors suggest that “uplift and subsidence are interpreted in terms of increase or decrease in fluid pore pressure in response to changes of the heat and gas flux. The subsidence during and following eruptions could be also linked with removal of material at depth to feed the eruptions”.
GNS conducts permanent volcanic monitoring and surveillance at all active and potentially active volcanoes in New Zealand.