Mr Johnstone (65) was holidaying in eastern Turkey during the flood and was immediately concerned about how much damage the flood caused given how much rain there was.
Since arriving home, he had used his experience to investigate the cause of floods to pick apart Dunedin City Council's actions before and after the flood.
However, the council yesterday disputed the conclusions Mr Johnstone had reached, saying they contained "serious factual errors''.
Mr Johnstone believed he would have spent upwards of 80 hours writing three reports on the cause of the floods.
"But I'm happy to do it, because I think the people of South Dunedin deserve a better deal.''
He was particularly touched by a conversation he had with a staff member at Farmers, who lived in South Dunedin, who told him she was "terrified'' every time it rained.
"That's an awful situation and there is no reason why they should be [terrified] if things were maintained properly.''
He also enjoyed the work.
"It forces me to exercise my background and knowledge and I enjoy the challenge of analysing data and making sure the conclusions people are drawing are actually consistent with the data.''
He planned to keep on monitoring what the council was doing and calling for a "proper'' maintenance regime to be put in place.
"If you don't accept you cocked up, it's very hard to fix things.''
Council Infrastructure Networks general manager Ruth Stokes said in a statement the council appreciated Mr Johnstone efforts.
"However, we believe his critique of the DCC reports contain serious factual errors and evidence that he just does not understand our network.
"Put simply, more rain fell in June 2015 than the South Dunedin stormwater network could cope with.
"Some of the DCC's infrastructure wasn't working as well as it should have and, as we have openly discussed, we are addressing these issues.
"However, with the amount of rain that fell, along with other factors such as water flowing into South Dunedin from the surrounding hills and a saturated ground, significant flooding was inevitable even if the system had been running perfectly.''
