The scientists used a method to estimate how often extreme floods and heavy rains might happen. They looked at data from a big storm in 1984 in Southland. They found that the heavy rain at Invercargill airport might only happen once every 840 years, with a possible error of 700 to 1000 years. The flood peak in the Waihopai River could happen once every 1900 years, with a possible error of 900 to 4000 years. These estimates match what was thought back in 1984.