I think you are right to be cautious. I don't know the answer but you could find out by contacting the Qld. Dept. of Primary Industries.
Cheers,
Dee.

Here's the relevant extract from the AQIS database:
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Condition C7119

Musa spp. (Banana) seed for sowing

Non-Commercial

The conditions under the Commercial section apply

Commercial

ImporterÂs Responsibilities

1. All consignments must be accompanied by a valid Import Permit or by means to allow the identification of the Import Permit.

2. It is the importers responsibility to check the requirements of local, state or any other government organisation prior to importation.

3. A Quarantine Entry must be lodged for each consignment.

4. The importer must contact the AQIS regional office in port of entry prior to arrival to confirm all arrangements for inspections and treatments of consignments.

5. The importer is responsible for payment of all associated AQIS fees and charges.

6. Seed must be free of live insects, soil, disease symptoms, prohibited seeds, other plant material (e.g. leaf, stem material, fruit pulp, pod material, etc.), animal material (e.g. animal faeces, feathers, etc.) and any other extraneous contamination of quarantine concern.

7. Each shipment must be packed in clean, new packaging, clearly labelled with the botanical name.

8. Seed must be either:

a). grown in closed quarantine at either an Australian Government (AQIS) post-entry quarantine facility, or a quarantine approved post-entry quarantine facility operating under a Compliance Agreement with AQIS;

or

b). used for generating tissue (embryo) cultures in a quarantine approved tissue culture laboratory.

9. The importer is responsible for contacting the post-entry quarantine facility to confirm all arrangements including space availability and number of seed lines, prior to the goods arriving in Australia.

10. There are scheduled fees associated with the growth of seed lines in an Australian Government (AQIS) post-entry quarantine facility. These fees are to be met by the importer. Refer to the AQIS website for further information on the fee schedule. State government post-entry quarantine facilities may have different fee schedules and importers are advised to contact the relevant state department for details.

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There's more, but you get the gist. Chances are that your seed would be intercepted by AQIS at Sydney airport (I think that's where all international mail gets screened), and impounded because it lacks the relevant paperwork. You would be given the option of paying to have it returned to the sender (something ridiculous, like $30) or having it destroyed if you did nothing by the given deadline. Same thing happened to me with a purchase I made, not knowing the seed was on the prohibited list. I assume my name is now on some database, probably on the list of suspected eco-terrorists :-)

Anyway, you end up with neither the seed nor your money.

Here is a link that might be useful: AQIS database