The biggest exporter, Gunns, has imposed rolling closures at woodchip plants, and logging contractors and forest workers who supply them are idle in a growing crisis for the industry.

The strong dollar and the global financial crisis were contributing to the slowdown in sales, said the chief executive of Gunns, Greg L'Estrange. But he said environmental certification for the chips was a concern, too.

Perceptions had grown that hardwood chips came from native forests that were not well-managed, Mr L'Estrange said. ''Certainly the brand has been damaged.''

Opponents say a shift in demand could hasten the end of the long conflict about native forests in Tasmania, where about 2000 hectares of old-growth is logged annually, Forestry Tasmania says.

''Most of the customers are demanding Forest Stewardship Council-certified wood right through the supply chain,'' said Paul Oosting of The Wilderness Society.

The certification requires that timber woodchips come from existing plantations, not old-growth forests assessed as having high biodiversity.

The Minister for Resources, David Llewellyn, recently joined the chairman of Gunns, John Gay, on a trip to allay the concerns of six Japanese woodchip customers.

Mr Llewellyn told reporters on his return he wanted to reassure the companies that Tasmania's practices were among the best in the world.

''I've informed them that we will actually be seeking, and I've asked Gunns and I've asked Forestry Tasmania to seek, certification under the Forest Stewardship Council,'' he said.

Mr L'Estrange said Tasmanian wood generally had the certification of the industry-favoured Australian Forestry Standard. ''For whatever reason, in some markets FSC does have a preference.''

The demand for rapid change was highlighted this week by a visit from one smaller Japanese chip importer, Mitsubishi, whose representatives told The Wilderness Society they aimed to take only plantation timber by 2012.

Gunns has confirmed that its Longreach woodchip mill, in the Tamar Valley, is to close for five weeks, after a similar closure at the Triabunna mill. A third mill is also operated at Hampshire, south of Burnie. The three have run on short weeks for several months.