"We emphasise in our public articles that stress testing results should not be read at face value," Orr said in a letter.
"Both the significant modelling uncertainties, and the fact that the banks know how/when the stress situation ends, limits the value of stress tests," Orr said.
Such caveats would always apply to any kind of economic modelling, but that doesn't stop economists – or the RBNZ – doing their darndest to try to make sense of the economy through such models.
Everybody knows reality will never precisely match the models, but they at least provide an idea of what might happen, and where the vulnerabilities might lie.
The puzzling thing about the RBNZ's last comprehensive stress test, conducted in 2017, is why its results are supposed to be so unbelievable as to require a doubling of bank capital.
The central bank "kitchen-sinked" that test, throwing every dire circumstance imaginable into the mix.
All four major banks, which account for about 88 per cent of New Zealand's banking system, sailed through that test.
One of the independent experts who reviewed the RBNZ proposals, James Cummings of Macquarie University, notes that "the capital ratios of major banks decreased to around 125 basis points above minimum requirements" as a result of that stress test.
In other words, none of the four came even close to eating into their statutorily required capital, and even had a reasonably healthy margin to spare.
To recap, these were the dire circumstances the banks were required to model: house prices plummeting 35 per cent, commercial property faring even worse with a 40 per cent decline in values, unemployment shooting up to 11 per cent and the Fonterra payout to dairy farmers averaging $4.90 per kilo of milk solids, below break-even for the average farmer.
On top of that, the regulator overlaid an industry-wide scandal relating to bad behaviour in mortgage lending, such as customers successfully suing the banks for poor lending practices and failure to abide by the Responsible Lending Code.
"Like previous stress tests, this exercise suggests the major New Zealand banks can, as a group, absorb large losses in a downturn while remaining solvent," the Reserve Bank concluded at the time.
Of course, it added the proviso that no stress test can predict the actual impact of a severe downturn.
But note that phrase "like previous stress tests." New Zealand's big four banks, ASB, Bank of New Zealand, ANZ, and Westpac, all owned by Australia's big four banks, have passed many tests, all comprehensively with capital to spare.
They were acknowledged as among the world's strongest banks through the GFC, although Orr is now rubbishing that fact, and effectively trying to rewrite history.
The notion that the Australian banks "sailed" through the GFC and saved New Zealand's economy "is just wrong. They didn't sail through. They were last cab off the rank. They were 100 per cent government guaranteed. New Zealand had about a $120 billion underwrite for the banks," Orr told Radio New Zealand.
He didn't mention that the banks paid insurance premiums for that guarantee, which was forced on the New Zealand government by the Australian government.
And the truth is the local banks didn't have a capital problem then. They had a liquidity problem.
That was because credit markets around the world seized up almost completely for a time and, back then, the big four banks had borrowed short in wholesale markets to lend long on mortgages and other loans in the mistaken belief that global financial markets would always remain liquid.
Orr says the RBNZ had to buy $8b of bank assets "overnight, almost blind" to provide bank liquidity and savagely cut interest rates. "They sailed through because everyone else was paddling like mad," Orr said.
The RBNZ has long since dealt with the liquidity question, forcing the banks to lengthen the tenure of their wholesale borrowing from offshore, which has fallen to about 22 per cent of funding from about 35 per cent when the GFC hit, and to rely more heavily on domestic deposits.
It's also worth remembering that the minimum tier one capital requirement for banks pre-GFC was just 4 per cent of risk-weighted assets. The minimum is now 8.5 per cent and the RBNZ is proposing to raise that to 16 per cent for the four major banks and 15 per cent for the smaller banks.
It's also worth comparing the conditions the 2017 stress test modelled to what actually happened in NZ during the GFC: house prices fell about 15 per cent, less than half the test scenario, and then recovered rapidly.
The unemployment rate peaked at 6.7 per cent – although it did rise above 11 per cent in 1991/92 following former finance minister Ruth Richardson's catastrophic "Mother of all Budgets."
And the Fonterra farmgate milk price fell from $7.59 a kilo of milk solids in 2007/08 to $4.75 the following year, but had recovered to $6.10 by 2009/10.
It did fall as low as $3.90 a kilo in the 2015/16 season – the average farmer needed about $5 a kilo to break even back then.
In fact, RBNZ ran a stress test in late 2015 to find out what would happen if the dairy payout fell to $3 a kilo of milk solids in 2015/16, remained under $5 until the 2019/20 season, and land prices fell 40 per cent over that period.
The conclusion: "The banking system is robust to a severe dairy stress test," the Reserve Bank said then.
So, are RBNZ's bank capital proposals a solution in search of a problem?
