If you disposed of the shares after the record date and before the return of capital was received, the return of capital is a CGT event separate from the CGT event on disposal. The cost base for the return of capital is nil. Therefore, you have made a capital gain from this event of $0.50 per share disposed.
For AGL shares you acquired after 19 September 1985*, use the table in How do I treat the capital gain? to decide which method to use to calculate the amount to include in your net capital gain or loss for the year of income that comes from the capital return.
* Shares acquired before 20 September 1985 are pre-CGT assets and you therefore disregard any capital gain or capital loss you make on them.