Blackham said at the time it was due to the "cry wolf effect".
"When the disaster doesn't happen, people stop believing it."
Non-weather authorities needed to moderate their warnings, he said.
"The psychology involved in warnings is complex. People are heavily influenced by each other to ratchet up the response."
Meanwhile, the country's government-funded weather forecasters are also coming under fire over sending out "confusing" and "competing" messages over severe weather.
MetService said last week it stood by its watches, and that it couldn't have done anything more to warn New Zealanders.
"We had weather warnings out for a considerable period of time and before it was a warning, it was in the severe weather outlook," MetService meteorologist Tom Adams said.
"We had social media, press releases and radio interviews, so every base was covered.
"Apart from going around knocking door to door, there is not much more we could have done."
However, WeatherWatch analyst Philip Duncan said the message could have been lost due to MetService competing against another government agency, Niwa, for "storm headlines".
"No other country on earth has two government forecasters, let alone two aggressive commercial ones, but New Zealand does - and we wonder why the 'official' messaging is now getting diluted and lost."
State-owned enterprise MetService is New Zealand's official provider of weather warnings and a range of government-funded forecasts.
But in recent years Crown research institute climate agency Niwa has boosted its forecasting unit, building up a vigorous social media profile and employing extra meteorologists who predict and comment on approaching extreme or notable weather.
"Niwa and MetService combined get over $150 million of tax funding each and every year and now Niwa and MetService commercially compete in the exact same space," Duncan said.
MetService should lead the fight with the government to stop Niwa weakening their own tax-funded warnings and competing against them, Duncan said.
"Our entire population is lower than Sydney's yet we have two government-owned commercial forecasters competing in the same space."
Earlier this year Minister of Research, Science and Innovation Megan Woods said she was going to look into the matter.
"I do intend to look closely at this and speak to Niwa and MetService to ensure there isn't undue duplication of roles and effort."