All tenders for aerial suppression in Australia on behalf of the states is done by the National Aerial Firefighting Centre (NAFC) which funded by the federal government.
Each state has their own individual requirements. Here in Queensland we have 4 AS350 B3's from Helicopters Australia on contract during the season (Usually Aug -Nov) and have other machine's available on an Ad-hoc basis.
Victoria, NSW, SA and WA also have contracts with overseas contractors such as Coulson, Erikson and Evergreen Aviation.
I'm a member of the Queensland Air Ops unit, so feel free to contact me if you require anymore info..
Cheers