Tasmanian Irrigation extended water sales on the Tamar Irrigation Scheme for a fortnight following less-than-expected demand, forcing the scheme to be either heavily redesigned or scrapped altogether.

However, a new scheme not in tranche three will go ahead after farmers in the Meander Valley support water sales over and above the threshold.

Tasmanian Irrigation chief executive Andrew Kneebone said the Greater Meander Scheme had been identified as an extra scheme to add to the list of existing tranche three projects and recently had achieved over its water sales threshold.

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The Greater Meander Valley scheme will be funded by an extra $5 million in Commonwealth Government funding for the organisation.

"We have just completed water sales for the Greater Meander Valley and for Sassafras-Wesley Vale schemes," Mr Kneebone said.

Greater Meander Valley received offers of about 14,000 megalitres of water, which is more than enough to ensure the scheme goes ahead.

Mr Kneebone said sales for Sassafrass-Wesley Vale were also supported, and it looked like that scheme would also reach its threshold target.

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Mr Kneebone said the Greater Meander Valley scheme had experienced strong demand in areas TI didn't expect, which was great to see such support.

That is in stark contrast to the Tamar Valley scheme, which so far has been able to meet its threshold for water sales.

That leaves it at risk of not going ahead or undergoing a heavy redesign.

However, Mr Kneebone said Tamar had some unique challenges other schemes don't.

"The problem with Tamar [Valley Scheme] is that we need a lot of extra pipes to get to where the demand is, so we need to get that level of water sales," he said.

Demand is strong for Tamar in the Hillwood area, but it's a fair way away from the water source used to supply the scheme - and with that comes extra cost.

"The demand for Tamar is out in the extremities, which will mean more pipes to get out there, something unique to Tamar than for other schemes."

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He said another issue with Tamar is that it doesn't traditionally have an irrigated area to supply it, which means many farmers are "on the fence" on whether to support it.

"It's a greenfields site, Tamar is not like Greater Meander or Sassafras-Wesley Vale, so there's a lot of farmers unsure about whether the investment is worth it."

He said TI was looking at a redesign of the Tamar scheme to propose to farmers, which would secure a smaller amount of water and less infrastructure if the water sales threshold is not reached.

A scheme redesign occurred for the recently opened Don scheme, and Fingal also didn't reach water sales. Ultimately Fingal was not viable and did not go ahead.

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