Hi,
I am in Oz and most companies once they hear Union.. Back off. You hopefully have access to union solicitors as part of your entitlements. Personally I would start off by explaining to your employer that you work reduced hoursdue to your medical condition and you cannot do more. Gage their response. If it's negative, document it get a doctors certificate and consider stress leave/union consult.

I was in a Union for 15 years here in Oz and this was the usual route for members.