According to the NZ Herald:
[Little] said 305,000 children were living in poverty.He selected the highest number he could find. Technically speaking, in 2014 there were 305,000 children living in homes that were below 60 percent of the median equivalised household income after housing costs as measured by 'moving line'.
But you can bet that if Labour ever regains power they will very quickly start looking for the smallest numbers they can find. For instance 90,000 children are living in households experiencing severe material hardship as measured by European Union standards.