Short term casual employees are covered by the unfair dismissal provisions if they are dismissed for an invalid reason.
One of the Invalid reasons include: because of temporary absence due to illness or injury
All casual employees, whether short-term or long-term, are excluded from the notice of dismissal requirements of the Act. This means employers can terminate the services of casual staff without notice.

This is an extract from Wageline website.

Good luck, what an icky position for you to be in.