Standard & Poor's has put New Zealand Post and its subsidiary Kiwibank on negative creditwatch.
Domestic mail volumes fell 4.5% in 2011, a trend which the ratings agency expects to continue at a similar rate this year.
It says NZ Post's AA- rating could be lowered in the next two years if the decline in mail volumes accelerates and is not offset by improvements to delivery networks.
S&P also expects the company to become increasingly reliant on more competitive earnings streams, like parcel and express courier deliveries, which add to the risks facing the business.
Standard & Poor's put Kiwibank on watch. While nothing's changed to affect the bank's rating outlook, S&P says it should be equal with that of its parent.
Both NZ Post and Kiwibank are owned by the State.
