I have been on sick leave for 4 months in a non-union administrative position. I have been continuously harrassed for being off sick. My medical absence note was denied and now my work is saying if I do not show up for my next shift It will be seen as a serious misconduct and I will be terminated.
How can they terminate me on sick leave?

They should not terminate your employment while you are on a valid medical leave. This would amount to a wrongful dismissal, in which case you would be entitled to severance pay. Additionally, this woukd anount to a violation of your rights under applicable human rights legislation. Please contact our firm to discuss the matter in more detail and determine how we can assist you.