Launceston real estate agent Harcourts announced the sale yesterday.

Harcourts Rural director John Hewitt said he believed it was the biggest offering of private water for sale in the state.

"We are expecting it to set a bit of a benchmark in Tasmania," Mr Hewitt said.

It was difficult to set a value on the water rights because they had not been traded before, he said.

"Irrigators are paying $1100 a megalitre off dams but these are not the same surety - they are lower surety," Mr Hewitt said.

The water rights are from rivers across Northern Tasmania with large rights on the South Esk and Arthur River catchments.

The licences vary from 16 megalitres to 15,860 megalitres.

A Launceston auction, on November 4, will deal with 10 direct-take licences while tenders will close on November 11 for interest in 24,917 megalitres in the Arthur and Emu River catchments.

Mr Hewitt said the balance of the water rights, mostly storage, would be offered for private sale in the coming weeks.

He said water rights did not have to be attached to property but Gunns' rights were mostly off plantation blocks the company was no longer using.

"This will enable other farmers to take up the water," he said.

Gunns shares continued to nosedive yesterday.

Gunns resumed trading at the start of this week after more than a month out of the market with an opening price of 20.5 cents a share.

Gunns shares closed at 15 cents yesterday - an all-time low - after more than 18.5 million shares were traded.