if you have a contract you are not a casual worker, unless it specifically says you are lol. i have a girl who just works for me 4 hours on a Sunday but she has just had 8 weeks off sick, and only the first day, which was classed as first day sick, went unpaid, as she has the same rights as any other employee.
noone can threaten anyone with dismissal without going thru a formal set of disciplinary procedures, if these people who are saying this to you are meaning it, they are breaking the employment laws.
On the other hand, get some bloody work done!" lol