One little-noticed consequence of the resources boom has been a big rise in the current account deficit on our balance of payments with the rest of the world. But when we get the latest figures on Tuesday we're unlikely to see any evidence of that. Why not? Because of the surge in household saving.
The current account deficit occurs because our imports and payments of interest and dividends to the rest of the world almost always exceed our exports and receipts of interest and dividends from the rest of the world.
Over the past 30 years the current account deficit has averaged about 4 per cent of gross domestic product. Since the start of the resources boom in 2003-04, however, the current account deficit has been nearer 5 or 6 per cent of GDP.
You might expect that, with the resources boom meaning the world is paying us much higher prices for our exports of coal and iron ore, the current account deficit would be smaller rather than larger. But it hasn't worked out that way.
Why not? Because the higher export prices represent an increase in the nation's real income, and when that extra income is spent by individuals and firms, much of the additional spending goes on imports.
But it's always easier to see the factors driving the current account deficit if we explain it in terms of saving and investment rather than exports and imports.
How is it possible for us to go on year after year having our recurrent payments to the rest of the world exceeding our recurrent receipts? It's possible only if we can cover the difference by having someone in the rest of the world lend us that difference (or by accepting foreign ''equity'' investment in Australian businesses).
These capital transactions are recorded in the capital account on the balance of payments. So it turns out that if we're running a deficit on the current account this has to be exactly matched by a surplus on the capital account.
And the capital account surplus represents the amount by which the nation's investment in new housing, business plant and structures, and public infrastructure during a period exceeds the nation's saving during that period.
Households save by spending less than all their income on consumption. Companies save by retaining some of their after-tax profits rather than paying them all out as dividends. And governments save when they raise more in taxes than is needed to cover their recurrent spending.
The amount we save pays for the amount we invest. So when a nation's physical investment spending during a period exceeds the amount it has saved during that period - as it always does in Australia - it has to cover the gap by calling on the savings of foreigners.
Looked at this way, the resources boom increases the current account deficit because it leads the mining companies - many of which are foreign-owned - to greatly increase their investment in the construction of new mines, natural gas facilities and so forth.
In the first stage of the resources boom - before the global financial crisis interrupted - there was an increase in national saving, but a greater increase in national investment, thus causing the current account deficit to be 1 or 2 percentage points of GDP higher.
Then there was the period of the crisis - particularly in 2009 - when national saving increased but national investment fell, thus causing the current account deficit to narrow to about 3 per cent.
But now we've got the economy recovering from the mild recession induced by the crisis. We have seen a bit of growth in investment in new housing, stronger growth in companies' investment in ''non-dwelling construction'' (mainly continuing construction of new mines), though no growth in companies' investment in new machinery and equipment, and very strong growth in governments' investment in infrastructure, particularly the state governments.
So national investment spending is up on the crisis period. But national saving is up by more. As we saw in this column last week, the household saving ratio has shot up to 10 per cent of household disposable income (equivalent to about 6 per cent of GDP).
Corporate saving is quite high, as companies use retained earnings to repay debt and improve their gearing. Yet governments have gone from saving to dissaving as their revenues have been hit by the delayed effect of the downturn while their recurrent spending has been swollen by stimulus measures.
Putting these three components together, national saving is well up on what it was before the crisis struck. Whereas gross national saving had coasted along each year at about 20 per cent of GDP (here, ''gross'' means before making a deduction for the annual depreciation in the value of the stock of the nation's physical capital), now it is up to about 25 per cent.
So, though national investment is up a bit on what it was during the crisis, national saving is up a lot - mainly thanks to the remarkable increase in household saving. This suggests the current account deficit, which got down to 3 per cent of GDP during the crisis, has taken a step lower to 2 per cent. It averaged 2 per cent in the June and September quarters of last year and, when we see the balance-of-payments figures on Tuesday and the national accounts on Wednesday, they're likely to show the current account deficit stayed at about 2 per cent in the December quarter.
Two conclusions. First, the fact that the increase in the current account deficit during the noughties occurred because of higher investment rather than reduced saving (which actually increased a bit), suggests that the foreign debt we are racking up is financially sustainable. In the main, we're borrowing from foreigners to expand our capacity to sell more coal, iron ore and natural gas to foreigners.
Second, the expectation that the resumption of the resources boom will lead to many more years of outsized current account deficits arises because we know there's a huge amount of investment in mining construction to come, with much of the funding for that investment coming from foreigners.
But this expectation assumes no change in the nation's saving habits. So to the extent that our households continue saving a lot more of their incomes than they used to, we can have the mining construction boom with lower-than-expected current account deficits and less increase in our foreign debt.
