Auditors are not, of course, experts on the state of the nation's drains. But they can compare numbers and when they look at the councils' accounts the results are troubling.
There is a huge gulf between the depreciation expense claimed by councils in their annual financial reporting, and the capital expenditure on renewing those assets budgeted for in their 10-year plans.
On average the latter is only two-thirds of the former, resulting in a cumulative shortfall by early next decade of $6 billion to $7 billion.
An investor looking at that sort of gap between depreciation and capex in an infrastructure company's accounts would not necessarily run a mile, but would have some pretty pointed questions.
Not only that. The Auditor-General's office found that between 2011 and 2013 the councils collectively underspent their capital budgets.
Nor are they collectively conserving cash for a spend-up to come. And their cumulative debt is forecast to increase by $6 billion by 2022, even with the gap between depreciation and renewals spending.
The gap between budgeted renewals spending and depreciation is narrowest for roads (9 per cent), where deterioration in the asset is visible. The fact that this spending is co-funded by central government and not entirely a charge on ratepayers may also be a factor.
By contrast, only a third of the depreciation of stormwater drains is being covered by budgeted renewal spending. In light of climate change, and the expected greater frequency and severity of flood events, that could prove very false economy indeed.
Clearly the depreciation expensed in the councils' accounts is not Holy Writ. The numbers will embody assumptions about the useful lives of the assets, their current state and their replacement costs.
But when there is an uncertainty band around these numbers, it is unwise to assume it will be resolved in our favour. Things might be worse, not better, than today's numbers portray.
Auditor-General Lyn Provost stressed the need for better and more consistent information for councils to use when making decisions on the management and funding of these assets.
Local Government New Zealand, the sector's national body, is in the throes of a major review of three waters infrastructure: a stocktake of the status quo and an analysis of the challenges ahead.
It needs to get a handle on how much of a real problem lies behind the prima facie issue the Auditor-General's aggregated results are pointing to.
"The data we have indicates the system is broadly in good shape today," says LGNZ chief executive Malcolm Alexander.
But looking out over a 20-year horizon they can see a bulge in the renewal curve, the echo of a wave of spending on all sorts of infrastructure during the 1950s and 60s - a sort of baby boom effect.
The International Infrastructure Management Manual's 2011 edition says, in the careful language of civil engineers: "Along with many other countries, New Zealand is facing a looming infrastructure renewal peak which is not well-defined in quantum or timing. What is known is that a significant amount of infrastructure was developed during the 1940s to 1970s and is reaching the end of its expected life.
"However, with much of this infrastructure underground, there is still much debate around what that life expectancy is. Funding the renewal of this infrastructure together with continuing to invest to meet growth requirements will be an ongoing challenge." Alexander says that following the Canterbury earthquakes, councils also have to consider whether like-for-like renewal is good enough, or whether a greater degree of resilience needs to be aimed for.
The background to their deliberations is changing demographics: an ageing, ever more urbanised population living in smaller households. The challenge traditionally has been to invest to cope with growth, but now much of the country faces the prospect of smaller and older populations.
Projections released by Statistics New Zealand last week indicate that - taking median projections for fertility, mortality and migration - 60 per cent of New Zealand's population growth out to 2043 will be in Auckland. That presents one sort of infrastructure investment challenge.
Conversely, 26 of the 67 territorial local authorities are projected to have fewer people by then than they do now. Declining rural areas face difficult choices about what infrastructure spending to undertake and how to fund it.
The issue is bedevilled by the mismatch in time frames between the optimal management of physical assets and councils' finances - over decades on the one hand and a three-year electoral cycle on the other.
And it may be that arguments in favour of providing decent infrastructure for future generations cut less ice at a local level than they once did, given how mobile people now are. But that is what long-term bond markets are for.
