You can file for a 1-year or 3-year multiple-entry business visa that will allow you to visit Thailand as much as you want while the visa is valid, and you will be allowed to stay for a maximum of 90 days per visit.
Furthermore, Thailand and Australia are both members of the Asia-Pacific Economic Cooperation (APEC) and participate in the APEC Business Travel Card (ABTC) scheme, you may be able to apply for an ABTC, which acts as a multiple-entry business visa that allows you to travel to 17 other APEC economies with only one card.