(a) That is not correct. Section 20(1) of the ACL states that: 'A person must not, in trade or commerce, engage in conduct that is unconscionable, within the meaning of the unwritten law from time to time'. What this means is that conduct that would amount to unconscionable dealing in the 'unwritten' law (that is, common law and equity) is also a breach of s 20. This creates an overlap between the general law and the statutory law.
Why create this overlap? Because under the general law, the only remedy for unconscionable dealing is to rescind the contract, making it void ab initio, and restoring the parties to their pre-contractual positions. This is not always a particularly appropriate remedy. The ACL provides additional remedies in the event of a breach of s 20. These remedies include the power to alter the terms of a contract that was entered into because of unconscionable conduct.
In the present case, B Pty Ltd has taken unfair advantage of a serious (albeit temporary) inequality of bargaining power and has likely breached s 20. On the facts in the example, the court would likely vary the agreed terms of the contract by reducing the agreed rental.