ASB Bank said a resilient New Zealand economy and low levels of bad debt helped the bank to achieve a record statutory net profit of $372 million for the first half to December 31.
The result was a 31.4 per cent improvement on the same period a year earlier, but the bank warned that the outlook for the second half would be clouded by Europe's economic woes.
A positive $48 million adjustment in the fair value of its derivatives, which the bank is obliged to do under "marked to market" accounting rules, was a factor behind the big increase in the headline profit number.
Excluding these movements from both periods, the statutory net profit would have grown by 12.9 per cent.
Asked if she expected to see a backlash over the bank's high earnings, ASB chief executive Barbara Chapman said: "The profit as a headline number does look large, but you do need to put that into a New Zealand context and we are very large businesses.
"Our return that we provide on assets of around 1 per cent is not enormous, and there are not many businesses that operate on those sorts of returns ... in any sector."
Chapman said ASB's performance was influenced by a combination of factors, including healthy revenue growth and productivity gains.
"More broadly, the strong result should also be seen in the context of the current low credit growth environment, which has seen constrained growth in lending and balance sheet size," she said.
Contributing to the performance was a fall in impairment losses of 61.1 per cent to $14 million compared with the same period last year, as a consequence of improving asset quality over all sectors, including business lending.
David Tripe, senior lecturer at Massey University's Centre for Banking Studies, said ASB's impairment loss was "incredibly low" and the first half's profit performance would be difficult to replicate in the second.
"It is unlikely that that level of profitability is sustainable because of the impact of the fair value adjustments, and it looks as though bad debt expenses are unusually low," he said.
ASB said a factor in its result was the ongoing trend among homeowners to switch from fixed to floating-rate home loans.
Chapman said the main area of resilience was in the business banking markets. She pointed to Reserve Bank data which showed business borrowing for 2011 was up 1.7 per cent compared with a 2.5 per cent fall in 2010 and an 8 per cent drop in 2009.
The ASB's net interest margin - the difference between what it costs the bank to borrow money and what it lends it out at - rose by 0.15 per cent to 2.19 per cent.
Meanwhile ASB's parent company, Commonwealth Bank of Australia, which has attracted negative press for raising interest rates despite an unchanged official cash rate, reported a statutory net profit of A$3.62 billion ($4.63 billion) for the six months - up 19 per cent.