Standard & Poor's has changed the outlook for its A+ credit ratings for Kiwibank and its state-owned parent, NZ Post, to negative from stable, saying there is a one in three chance of a rating cut in the next two years.The ratings agency said the outlook change was because of rising economic risks in New Zealand, where a surge in house prices in the two largest cities of Auckland and Christchurch has sparked fears a future housing downturn could hit bank credit quality. Last week, S&P downgraded the outlook for three other smaller New Zealand banks, TSB Bank, Heartland Bank and The Co-op Bank, because of similar concerns about New Zealand's rising net foreign debt and reliance on foreign funding. NZ Post's outlook was reduced because Kiwibank now produces 70 per cent of its earnings and the national mail service faces falling letter volumes.S&P said its stand-alone credit profile of BBB for NZ Post was based on the "very high" likelihood that the New Zealand government would provide timely and sufficient extraordinary support to NZ Post in the event of financial distress."Questions about government backing for state-owned enterprises have arisen in recent weeks after the Government said it expected New Zealand's four biggest banks - NAB's BNZ, CBA's ASB, ANZ and Westpac - to take haircuts on nearly NZ$400 million of unsecured loans made to Solid Energy. The state-owned coal miner faces a restructure after a slump in coal prices plus an ill-fated plan to expand into other resources wiped out its remaining equity.S&P noted it was watching the government's level of support for NZ Post."Although unlikely in the next two years, downward pressure on the ratings may also arise if our assessment of the likelihood that the New Zealand government would provide timely and sufficient extraordinary support to New Zealand Post were to weaken."Fitch affirmed Kiwibank's rating at AA on Monday and left its outlook at stable. However, Fitch described Fitch Kiwibank's "capitalisation [as] modest."