New Zealand Post will not be able to supply much more capital to Kiwibank, Fairfax Media reported yesterday.Michael Cullen, chair of the board of NZ Post, wrote in a letter to state-owned enterprises minister Tony Ryall that the postal monopoly had "some small capacity" to supply more capital to its 10-year old banking subsidiary."Our preference would be for the Crown to inject capital, providing we are able to satisfy them that they will get a long-term return on that capital, which we believe they will, and that it fits with the Government's overall economic strategy," Cullen wrote.Fairfax obtained the letter under the Official Information Act. It did not publish its date.NZ Post contributed NZ$50 million in capital to Kiwibank in the second half of 2011 to take share capital to NZ$360 million. With retained reserves and hybrid capital provided by private investors, the bank had NZ$699 million of capital resources at the end of December 2011.NZ Post has used Kiwibank to provide an alternative use of its network of more than 800 outlets, and as a source of revenue in an era when the traditional letter is in decline.The main thrust of Cullen's letter to his minister is to emphasise how fast the traditional business of NZ Post is changing, with letter volumes falling quickly.Fairfax did not cite any reference to the increase in parcel deliveries arising from the boom in offshore purchases from internet-based retailers.