In Aus, Transition Altitude is A100, (10,000') on QNH. Transition Level is FL110, (11,000') on 1013.2 hPa.
At a QNH at or above 1013.2, there is a minimum of 1000' vertically between the Transition Altitude and the Transition Level.
In an area of low pressure, FL110 is not available for cruis flight, to preserve the 1000' buffer between aircraft at A100 and aircraft above A100.
As the QNH drops, FL115 is not available if QNH < 997 hPa, FL120 is not available when the QNH < 980 hPa, FL125 is not available when the QNH < 963 hPa, and so on.