An Australian union representing forest workers has warned that three ministers will be ousted at the federal election if old-growth logging is banned in Tasmania.

The Construction, Forestry, Mining and Energy Union is spending A$20,000 on a television advertising campaign to back timber industry candidate Peter Kelly in the interstate electorate of Gippsland.

Meanwhile, Tasmanian timber company Gunns has said speculation about forestry in the state has made investors nervous about spending money on forest projects there. It said any changes to the regional forest agreement would endanger the development of the Tasmanian economy.