Soil tests conducted by archaeologists and Aboriginal heritage officers where the bypass is planned to cross the Jordan River, north of Hobart, suggest stone artefacts encased more than 30centimetres below the surface could be up to 40,000 years old.

"If the ages for the site prove to be correct it would be the oldest most southern site on the planet, giving us a glimpse into an unknown part of world history and the spread of homo sapiens across the earth," a briefing note sent to the Aboriginal community and the Infrastructure Department says.

"It will be an important place for interpreting the deep history of Tasmania but also archaeology on a worldwide scale."

The archaeological team believes up to 3million artefacts could be buried on the 600-metre by 60-metre levee.

The age of already- excavated artefacts will be confirmed within the next six weeks.

However, Infrastructure Department secretary Norm McIlfatrick yesterday moved to preserve the area, unveiling plans to redesign the bridge so it did not disturb the site.

Mr McIlfatrick said the redesign could add an additional $5 to $10million cost to the $176million project but was not anticipated to delay the finishing of the project.

Tasmanian Aboriginal Centre legal director Michael Mansell said the road should be diverted 300metres north along the existing railway line.

"The machinery involved in the construction of the bridge could alone destroy the site," he said.

He called on Heritage Minister Michelle O'Byrne to declare the levee a protected site under the Aboriginal Relics Act.