is culling animals for food illegal in Australia?

Yes, legal. But, for them to sell it as processed meat, it probably needs to be inspected and slaughtered at an approved facility.

In other words, you can buy the live animal and slaughter it yourself. Otherwise, they must take it to a proper slaughterhouse.

Sometimes farms will sell the animal "live" and offering free processing to get around this requirement.

Do you even understand what farms are for? To raise crops and livestock for food.