Westpac suffered a shocker of a year in New Zealand on mushrooming bad debt charges and a massive tax bill but the extent of its misery has been obfuscated by what banking analyst David Tripe has labelled "scurrilous, nonsensical data".

The bank reported September-year cash earnings of $236 million, down 40 per cent on a year ago. The six months to September were particularly painful with cash earnings of just $34 million against $240 million for the same period a year earlier.

Westpac NZ chief executive George Frazis said the "disappointing" result was generated by impairment charges more than trebling from $170 million a year ago to $572 million, "mostly related to commercial property exposures".

Parent Westpac Banking Corporation, which yesterday indicated it would pick up the tab if its New Zealand division's appeal against a $961 million Inland Revenue "structured finance" tax bill failed, said the local operation had suffered "a challenging and disappointing year given the weak environment."

Westpac NZ did not publish a separate net profit or "bottom line" figure which includes non-cash items such as the now-standard revaluations of financial instruments.

"When banks report cash earnings it really is the equivalent of net profit after tax," Frazis told the Business Herald.

"From the shareholder perspective it's exactly equivalent."

However recent results from Westpac rivals BNZ and ANZ National show there is indeed often great variance between cash earnings and net profit figures.

BNZ reported cash earnings of $420 million but a bottom line net loss of $181 million. ANZ reported underlying earnings of $628 million but a net profit of just $194 million. Both banks' bottom lines were affected by big provisions for back tax and interest related to their own contested "structured finance" tax cases, but also by one-offs and non-cash items.

Frazis said if non-cash items had an impact on profit "they're already in there" as were any one-offs of which "we've got very few". However, Massey University's head of banking studies Tripe commented that an initial look at the figures released yesterday revealed glaring inconsistencies when compared with the bank's most recent General Disclosure Statement - a document the Reserve Bank says is "intended to assist bank customers to understand more about the financial condition of banks operating in New Zealand".

Yesterday, Westpac said bad debt charges for its New Zealand operation, excluding institutional banking, were $184 million for the March half. In its March GDS the corresponding figure was $338 million.

"It bears no relationship to the published financial data for the entities in question, what's the point in releasing it?" asked Tripe.

"My understanding is the Institute of Chartered Accountants is supposedly doing something to deal with this sort of scurrilous nonsensical publication of data. They obviously haven't got there yet but if they have, they haven't told Westpac."

Westpac later told the Business Herald the discrepancy arose because the March New Zealand GDS numbers included more recent - and significantly worse - bad debt numbers than those used to prepare its March-half Australian statutory accounts even though both sets of figures dealt with the same reporting period.