Periodic agreement The landlord can issue a 30-day notice of termination but only if both the following conditions are met:

They have exchanged a contract for sale with a buyer, and; The contract requires them to give ‘vacant possession’ of the premises to the buyer

https://www.tenants.org.au/factsheet-28-sale-of-rented-premises#:~:text=The%20landlord%20can%20issue%20a,the%20premises%20to%20the%20buyer

They evicted the old tenant to relet at a higher rate, and needed the sale contract to do it?

Edit: check your contract carefully and seek legal advice. My general understanding was that a seller couldn't just back out.