Jackson owns a 3-bedroom house in Newtown which he leases to 3 students on a 12 month lease from 1 January to 31 December at a rent of $900 per week. The lease has a clause prohibiting the tenants from sub-letting the premises without Jackson's express permission.

The students decide that while they holiday overseas in the July break, they will offer the house on airband.com at a rate of $300 per night. They have moderate success on airbandb.com throughout July and are happy that the income subsidises their holiday costs. (The students paid rent in full to Jackson while they were away.)

Jackson discovers what they have done and is incensed that they made money out of his property. He terminates the lease and the students depart by the end of August. However, Jackson then finds it difficult to find new tenants and the house is empty for all for September, October and November, until a new tenant Susan moves in on 1 December. (The rent under Susan's lease is $900 per week.) Jackson did not attempt to let the house on airband.com or other such sites while he looked for a permanent tenant.

Jackson includes terms in Susan's lease that a) subletting is prohibited and if Susan does sublet through airband.com or similar sites, Susan 'will pay to Jackson within 10 days of receipt 100% of the difference between the rent and the proceeds she receives' (clause X) and b) that if Jackson terminates the lease for any breach by Susan, she will 'remain liable to pay rent for the balance of the term of the lease as if she had remained in possession less any rent received under a new tenancy' (clause Y).

Jackson seeks your advice on whether these terms would be enforceable to any extent.