"Each reaction ... he 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 also repeatedly named the alleged gunman.
"Let's move on as a nation, never forget or forgive but strive to live as normal."
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.
Dr Mohsin said a number of people had spoken and written to him, in particular refugees, saying they had been offended by Casson's comments.
"I've just sent him this information that it would be good and in the community's interest that you post that on your Facebook page and then your visit to the mosque will be easier."
"We are a very forgiving community ..."
Casson, an immigration officer at Immigration New Zealand, declined to comment further at this stage after seeking independent advice advising him against it. However he said councillors were entitled to their own opinions.
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.
"During Cr Casson's elected term he is subject to our Code of Conduct, as are all elected members. We have assessed Cr Casson's comments against the Code of Conduct and sought independent advice. While his recent comments are not reflective of the views of many in our community or the Council, they do not breach the Code."
However Briggs said Casson's views did not reflect the view of the council.
"We are proud of the way the people of Hamilton have stood by Christchurch and the wider community since the attacks on 15 March."
Since Casson's post went live other elected members have taken to social media to say they disagreed with him.
Councillor Angela O'Leary said she did not share his views and agreed with the "deep concern expressed regarding Cr Casson's reflections following the Christchurch tragedy".
O'Leary went on to say Hamilton was made up "of the colour of all of our faces, our different cultures, our views and values, our ideas and our politics, our rainbows and our genders".
While councillor Dave Macpherson said Casson gave Hamilton a bad name and that his views deflected the positive inclusive direction council as a whole had been taking.