Working beyond your scheduled finish time is regarded as overtime. Whilst you can be ordered to remain on duty you cannot be ordered to work unpaid overtime. You should be compensated for your overtime at the appropriate rate and the decision as to payment or lieu time is yours.
You are compensated for each completed period of fifteen minutes and the rate currently stands at time and a third.
If you were informed at the beginning of a shift then this is pre planned overtime and you will be compensated for each period of fifteen minutes.
If not thus informed then this is casual overtime and you will not be compensated for the first thirty minutes of any overtime.