According to Brian Perry, the the expert at the Ministry of Social Development writing in last year’s Social Report:
The primary measure is the proportion of people in households with equivalised disposable income net-of-housing-costs below a threshold set at 50 percent of the 2007 household disposable income median – and held fixed in real terms (the 2007 anchored or constant value measure, CV-07).
This measure shows whether the incomes of low-income households are rising or falling in real terms, irrespective of what is happening to the incomes of the rest of the population.
The two other measures use fully relative thresholds set at 50 and 60 percent of the current year’s household disposable income median net-of-housing-costs (REL 50/60). These measures reflect how low-income households are faring relative to middle-income households.