Full-year profit growth from the New Zealand arms of ANZ, Westpac and NAB was more than triple that of their parents because of higher net interest margins, lower bad debt expenses and solid lending growth.A rebounding housing market in the country's biggest city, Auckland, and the start of the rebuilding of Christchurch helped boost the New Zealand economy, with household lending in particular contrasting with weak lending growth and the subdued housing markets of Australia. A structural shift away from less profitable fixed mortgages to more profitable floating rate mortgages boosted net interest margins across New Zealand's banks, whereas net interest margins in Australia fell as funding costs grew faster than they were passed on.NAB's BNZ arm reported cash profit growth of 22.6 per cent, to A$575 million, in the year to September 30, outperforming a slight group cash profit fall of 0.5 per cent. BNZ's net interest margin rose nine basis points to 2.39 per cent, while NAB's personal and business banking net interest margins in Australia fell 16 and nine basis points respectively.Westpac's New Zealand division grew cash profit 22 per cent to NZ$548 million, while group cash earnings rose five per cent. New Zealand's cash earnings growth of NZ$106 million compares well with the Australian Financial Services division's growth A$186 million, despite operating income for AFS being A$12.09 billion compared with A$1.56 billion in New Zealand. Westpac's net interest margin rose eight basis points, while AFS' net interest margin fell three basis points.ANZ's New Zealand operations grew underlying profit 11 per cent for the year, to NZ$957 million, while group profit rose six per cent. ANZ's net interest margin rose 10 basis points for the year, while the Australian division's net interest margin fell eight basis points. CBA's bank in New Zealand, ASB, which has June 30 financial year end, was the only outlier, with its cash earnings growth of four per cent being in line with that of the group overall.The financial year has been dominated by the relative out-performance, in market share terms, of ANZ in Auckland, the city with the biggest and most active housing market. ANZ moved to build up momentum ahead of its September announcement of the integration of National Bank customers into the ANZ brand, which was associated with the shift of ANZ customers and accounts over to the National Bank computer system. ANZ spent heavily on marketing through late 2011 and through the first nine months of 2012. This was largely linked to ANZ's sponsorship of the Rugby World Cup and New Zealand's Olympic team.From March 2012 onwards, ANZ reported that its share of account-switching in Auckland doubled to 40 per cent, and that its share of new mortgage registrations in Auckland had increased from 25 per cent to