The company said its shares will "better reflect fundamental value" once investors complete their reweighting.
"We remain positive about our future with recent growth in customer numbers, further gains in portfolio flexibility in the next two years, the delivery of the Te Mihi geothermal power station and a diverse range of options available for development as market signals dictate," Contact said in its letter.
The changes will reduce the market capitalisation of the NZX50 by $3.2 billion to $35.98 billion, NZX says.
The new methodology will also have a bearing on the weightings of the four state-owned energy companies being taken public in a sell down that will leave the government holding just over 50 per cent in each.
By contrast, Sky City Entertainment Group's weighting will rise to 6.1 per cent from 5.6 per cent. The casino and hotel operator doesn't have a significant cornerstone shareholder.
Fletcher Building will remain the largest listed company in the index, at a weighting of 12.5 per cent, up from 11.5 per cent at present. The new methodology will apply to all equity indexes from June 15, and an updated document will be released next month.