Commissioner of State Revenue (Vic) v Royal Insurance Australia Ltd [1994] HCA 61

RAI Insurance accidentally overpaid approximately $2 million in duties to the Commissioner of State Revenue. This was because RAI was unaware that the statute had changed. The Commissioner was aware of this.
It is worth noting that RAI had passed the costs of these duties on to their customers. Therefore, they did not actually feel the loss.
RAI Insurance requested that the money was refunded. The Commissioner refused, stating that she had a discretion to do so under s 111 of the relevant Act which stated “Where the [Commissioner] finds … duty has been overpaid… he may refund … the amount of duty found to be overpaid.”

It was held by Brennan J (with McHugh and Toohey JJ concurring, and Mason CJ agreeing in a separate judgement) that it was true that the section did give the Commissioner a discretion.
Simply that the money was owed was not enough to compel the Commissioner to give the refund
However, in this case, the Commissioner was under an obligation elsewhere in the law, under the general law of restitution. In these circumstances, the Commissioner is compelled according to uphold that law – and s 111 gives the mechanism for that.
Therefore, unless a defence was available, the Commissioner must refund the money.
The Court held no defence (including that RAI had “passed on” the costs) was available to the Commissioner.
Once it was accepted that the Commissioner should have refunded the money, then it becomes clear that mandamus is the correct relief.
Mandamus compels a public officer to follow through on their duty – in this case, processing the refund.
RAI succeeded on all counts.

This decision is significant because it shows that even a discretion that seems unfettered may not be if you look at the wider statute and law.
It is also a good example of when mandamus is the appropriate remedy.

“Therefore, if the facultative term “may” is used in the creation of a power, it does not in itself impose a duty to exercise the power but such a duty may be found in the statutory context in which the power is created” (emphasis added) (Brennan J)
“[The Commissioner] is under no duty to make a refund unless there be an antecedent liability to do so” (Brennan J)
“The Commissioner is a public officer vested with a power to be exercised for … discharging her liabilities. When the power exists and the circumstances call for the fulfilment of a purpose … but the repository of the power declines to exercise the power, mandamus is the appropriate remedy even though the repository has an unfettered discretion in other circumstances to exercise or to refrain from exercising the power. Mandamus will go where there is a duty to pay money” (Emphasis added) (Brennan J)