Kiwi Group's half-year profit fell because of costs from a troubled IT project and earthquake costs.
Its profit in the six months ended December was $44 million, compared with $65m the previous year.
Acting Kiwibank chief executive Mark Stephen said lending growth slowed, but had risen back to more usual levels.
"Lower funding costs and market pricing opportunities enabled Kiwibank's net interest margin to improve, lifting net interest income," he said.
Operating expenses rose by nearly a quarter to $224m because of costs from scrapping an upgrade to its IT systems, as well as quake costs.
Customer deposits grew by 3.9 percent, ahead of growth in loans to customers.
Kiwi Group, which includes Kiwibank and Kiwi Wealth, is owned by New Zealand Post, ACC and the New Zealand Super Fund.
NZ Post's half-year result was affected by declining letter volumes.
It made a small profit of $6m in the six months ended December, but if its stake in Kiwi Group is removed, there was a loss of $13m.
That compares with a group profit of $89m the year before.
Revenue fell 3 percent to $452m because of a fall in letter volumes, despite growth in parcels by nearly 10 percent.
Nearly 38 million fewer letters were delivered in the half, when compared with the same time the year before.
NZ Post chief executive David Walsh said the decline was disappointing.
"This is a significant challenge for NZ Post, and cannot be underestimated in terms of loss of revenue as we seek financial sustainability for this valued service," he said.
Mr Walsh said Christmas was a very busy time for parcels.
"Areas of focus for the second half are the ongoing need to make the letters business financially sustainable, maximising the opportunities from continued growth in parcels, and furthering plans for e-commerce partnerships," he added.