"Each reaction [gunman] sits in his cell in the smug comfort of knowing he has impacted on everyday Kiwi life."
In his post, which has since been deleted, he repeatedly named the alleged gunman.
He also made a post on his personal Facebook page in 2016 prior to being elected calling refugees scum after the terror attacks in Nice, France.
Casson, who is also an immigration officer, last night posted the apology as requested by Muslim leaders, saying he was not a racist person.
"Good evening. I know that my previous posts have caused offence to the Muslim faith and wider public which was never my intention. I offer my apologies to all.
"I am trying to make this right and have arranged to visit the Boundary Road Mosque to speak to Dr Asad Mohsin and all attending Muslim faith, not to ask for forgiveness but to offer my sincere apologies and hopefully explain myself.
"People who know me know I am not a racist person."
Meanwhile the council he sits on has distanced itself from his comments, saying they are in no way shared or endorsed by Hamilton City Council.
Hamilton City Council chief executive Richard Briggs said the council had received a number of complaints about Casson's comments, but would not be doing anything about it as they did not breach the city's code of conduct.