Duties Act 1999

If after an agreement for the sale or transfer of dutiable property is entered into and before the property is transferred—

the consideration under the agreement is reduced and the reduced consideration is not less than the unencumbered value of the dutiable property when the consideration was reduced; or

the consideration under the agreement is reduced because the parties have agreed not to transfer some of the dutiable property previously agreed to be transferred and the reduced consideration is not less than the unencumbered value of the dutiable property that remained to be transferred when the consideration was reduced; or

the consideration under the agreement is increased and the dutiable value when the consideration was increased is greater than the dutiable value when the agreement was entered into;

The liability to pay additional duty arising from an increase in the consideration occurs on the day when the consideration is agreed to be increased.