The true extent of swamp kauri being sent overseas has been revealed - with exports jumping by more than 2500 per cent in five years.

The Government moved to strengthen the oversight of swamp kauri extraction and export last month after heavy criticism by opposition parties and conservation groups.

Allegations included officials turning a blind eye to dubious export practices, and ecologically-sensitive wetlands being wrecked in the "black gold" rush.

Now, newly-released figures show how rapidly the controversial industry has grown - and who is behind exports.

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In 2009, 173 cubic metres of swamp kauri was legally exported. That increased to 1150 cubic metres in 2013, before sky-rocketing to 4356 cubic metres last year.

About 800 cubic metres have been sent offshore so far this year.

One company, Silver Fern Resource Trading, sent 1810 cubic metres last year - all to China.

The Forests Act 1949 bans the export of swamp kauri logs, many of which are tens of thousands of years old, unless they are made into finished timber products.

Whole or sawn salvaged swamp kauri stumps or roots can be exported with a Ministry of Agriculture and Forestry milling statement and export approval.

Opponents of the trade, including the Far North Protection Society, say exporters are skirting the ban by labelling kauri slabs as table tops, or superficially carving the logs and calling them artworks.

New photographs, released to the Herald under the Official Information Act, show the sparsely-carved kauri logs that were approved as Maori carvings.

The Ministry of Primary Industries is now investigating whether the same logs have remained in their exported state, "as part of routine compliance".

However, only three "temple poles" have been exported since 2012 - the vast majority of exports are of products approved as finished such as tables, or raw stump material.

Last month, Primary Industries Minister Nathan Guy announced changes that included requiring operators to notify 100 per cent of all finished products for export approval, which MPI would inspect.