If your medicine is not banned in Australia, you can put it in your checked baggage and bring it in aboveboard, but you have to declare it at the entry point. You don't need to know the ingredients of the medicine you bring. They will read the instructions themselves, and half of them are in English, but you can say it's diet pills, so they shouldn't embarrass you. Don't be lucky not to declare. Once you find something to declare but don't declare it, you will be fined and even bear legal responsibility.

If yours is not allowed to be brought into Australia, I suggest you don't take the risk of stealing columns, because in Australia, drug dogs are used to sniff everyone's packages now, and no matter how tight you are, you can't escape the dog's nose. If you are found to be in possession and illegally entering the country, your crime will be even greater.

I suggest you have a look at the list of prohibited items in Australia, which can be found online. Personally, if your diet pills are not as many as 100, and you obviously want to sell them, they will let you take them in at a glance. Don't worry.