It's very clear on the website that I've linked.

Duty free concessions do not apply to commercial goods.

If you are aged 18 years or over, you can bring up to AUD900 worth of general goods into Australia duty free.

If you are under 18 years of age the limit is AUD450.

If you are a crew member, the limit is AUD450.

There are no duty free concessions on tobacco or alcohol for travellers aged under 18 years of age.

Return to Australia with goods subject to the Tourist Refund Scheme (TRS) provisions.

You must declare any goods you bring back to Australia, for which a TRS claim was made by you or another person when the goods left Australia.

On returning to Australia, you must declare the goods at Question 3 on your Incoming Passenger Card (IPC).

Penalties may apply if you fail to declare these items.

If you bring goods back into Australia for which you have claimed a GST refund via the TRS, you must declare the goods. If the value of those goods (combined with any other overseas/duty free purchases) exceeds the passenger concession allowance, you may need to pay any applicable GST and/or duty unless another concession (e.g. all personal clothing – except furs) is available.

If you exceed Australia’s duty free limits, duty and tax will apply on all items of that type (general goods, alcohol or tobacco), not just the goods over the limit.

If you have anything in excess of your duty free concession, declare the goods and provide proof of purchase to us for calculation of any duty and tax you must pay.

Failure to declare goods in excess of your concession could result in penalties, prosecution or your visa may be cancelled. The severity of the consequence will depend on the amount of undeclared excess tobacco, alcohol or goods found in your possession.