A five per cent NAIRU, that's what we're working with, federal treasurer Scott Morrison explained to 2GB late last week.The recent decline in the unemployment ratio may be a worrying trend for those with a fix on a natural rate of unemployment."We're still, as an economy, at 5.4 per cent" unemployment, Morrison told interviewer Ross Greenwood."You're right, that's the best unemployment rate we have seen since the start of 2013."Morrison rowed for a divisive economic idea."That's still 0.4 of a per cent above what you'd call the full employment rate, which economists refer to," he said."Once you go below unemployment of five per cent, it's what they call the non-accelerating inflation rate of unemployment which means your inflation starts to grow because your labour market is even tighter than it would seem to be at full employment. "So, I don't know if that's the best retail explanation I've got."