You may be in a spot of trouble if you ever really wind up having to kill two birds with the throw of one stone. It is a tricky shot to pull off. We shouldn’t put the Reserve Bank into that situation.

Labour’s Grant Robertson proposes expanding the Reserve Bank’s mandate to include unemployment. Last year, Robertson speculated about widening the Bank’s mandate even further, to consider the exchange rate, employment, wages and ‘the overall health of the economy’.

Monetary policy can only reduce unemployment in the short term. In the longer term, unemployment comes back and the country is stuck with a higher inflation rate. A central bank facing a dual mandate can be tempted into running too hot during asset price booms, and investors will know it.

As Oliver Hartwich reminds me, Germany’s Helmut Schmidt preferred five percent inflation to five percent unemployment - and eventually got both. Chasing a dual mandate can do harm to both objectives.

Similarly, central banks can get in trouble when trying to chase two targets with one instrument. The Reserve Bank can use the OCR to target inflation rates. Trying to use the OCR to target both inflation and unemployment requires that all stones hit two birds. It’s a heroic ask – especially when hitting even a single bird has proved a bit tough over the past decade.

If your hope is that the Bank would sensibly prioritise inflation under a dual mandate, think again. Full employment does follow naturally under stable inflation. But if that is what a government proposing a dual mandate wanted, no change to the Policy Targets Agreement is needed. The Bank already aims to avoid unnecessary instability in output (unemployment) while pursuing stable inflation. A dual mandate would tell the Bank that the government wants it to lean more heavily on short term unemployment.

Robertson’s attack on inflation targeting is especially odd in the current environment. Today’s 5% unemployment rates reflect strong growth in the labour market. Workers are being drawn into the labour force rather than abandoning job searches and leaving the labour market. And New Zealand’s employment rate is the fourth highest in the OECD.

The Reserve Bank has plenty to worry about in dealing with the consequences of a broken housing market. Trying to make trick shots firing its OCR stone at two birds at the same time seems ill-advised.