Mr Howard played down the assessment that the drought was the worst in 1,000 years, saying he doubted if anybody really knew
So: a 1000-year drought (or indeed a 1000-year anything) is a statistical event. What you’re saying is “you’d expect an event of this magnitude on average once in a thousand years”.
“The worst drought for 1000 years” is quite different. That statement implies that you have 1000 years of data on droughts, you have analysed the data and the current drought is the most severe in that timespan.
What the scientists are trying to say is: the current drought is unexpectedly severe, by a long way. I hope that’s now clear.