Fitch Ratings says Kiwibank's fast growth could put the financial health at risk.
The ratings agency has assigned the State-owned bank a AA long-term rating, with a stable outlook, saying the support from its parent, New Zealand Post, is extremely strong.
While the quality of its assets remains sound at the moment, Fitch warns fast growth could pose a risk, and potentially affect its profit and capital base.
Fitch says Kiwibank's strong loan growth over the last three years has outpaced deposits, making the bank more reliant on wholesale funding markets, which has weakened its funding mix.
The ratings agency says the bank's moves to grow deposits should help, as will its plans to issue covered bonds.
Covered bonds raise money against a pool of mortgages which are ring-fenced from other assets on a bank's balance sheet, they are used widely by the big Australian-owned banks, and in Europe.