Another reason not to expect that central banks can keep actual output growing at a rate exactly equal to the trend growth in potential output is that monetary policy affects demand, and hence inflationary pressure, with a considerable lag - and a lag which varies from economy to economy and from one period to another.
And within the period between the time a central bank adjusts monetary policy and the time that policy adjustment affects output, and the further time before the change in output affects inflation, all manner of things may change in ways which are difficult or impossible to predict - the world economy might change (of huge importance to economies like New Zealand and Australia), the economy might be affected by a drought, government might change its taxation or spending policies, government might change its immigration policy, and so on.