You can advertise a commercial listing simply by uploading it to various portals where relevant.
When creating the listing from the Properties screen, be sure to select 'New Commercial Listing' from the drop-down list.
Once the commercial listing is created and you are ready to upload to portals, please note the Australian portals realestate.com.au & domain.com.au manage sub-sites which specialise in commercial property. Those sites are realcommercial.com.au and commercialrealestate.com.au respectively.
You don't need to have a separate portal set up to use these, providing your subscription with Realestate.com.au and/or Domain.com.au includes commercial listings. If you're unsure about whether this is the case, we'd recommend reaching out to them both to confirm.