If you have taken paid personal leave (sick leave) and your employer requests that you provide a medical certificate, then you will need to comply.
The Fair Work Act 2009 (Cth) (FW Act), section 97 allows you to take paid personal/carer’s leave when you are not fit for work because of a personal illness, or personal injury, affecting the employee; or when you need to provide care or support to a member of your immediate family or household, who requires care or support because of a personal illness, or personal injury, affecting the member; or an unexpected emergency affecting the member.
However, under section 107 of the FW Act you must give your employer notice as soon as possible and provide evidence if your employer requires it. ‘Evidence’ must be evidence that would satisfy a reasonable person. In most instances a medical certificate is suitable evidence.
A modern award or enterprise agreement may include terms relating to the kind of evidence that an employee must provide in order to be entitled to paid personal/carer’s leave, unpaid carer’s leave or compassionate leave
If you do not provide evidence as requested, then your employer may discipline you which could lead to the termination of your employment.
If you are terminated and you have been employed with this employer for fewer than 6 months, then you will not be able to lodge an unfair dismissal claim however you might be eligible to lodge a general protections claim.