Massey University banking lecturer David Tripe says a downgrade of New Zealand Post's credit rating could increase the interest rates Kiwibank has to pay to fund its mortgage lending.
Standard & Poor's cut the ratings on Tuesday for both NZ Post and its banking subsidiary by one notch to A+ with a stable outlook
Deposits and loans by investors to Kiwibank of $13.4 billion were guaranteed by New Zealand Post at the end of June.
The agency says the size of that guarantee, which has grown with the increase in the bank's lending book, coupled with the group's struggling postal business, led it to cut the ratings.
Mr Tripe said the downgrade has the potential to increase the interest rates Kiwibank pays to fund its own lending.
The Government pledged $300 million of uncalled capital to New Zealand Post two years ago to support the group's credit rating.
Mr Tripe said the growth in Kiwibank's lending book are again raising questions about how Post can back its subsidiary's growth without going back to the Government for more capital.
Separately, New Zealand Post says it is continuing to work on strategies to boost its digital business to offset declining letter and parcel volumes and to increase the size of its banking operations.