Marlborough's District Council has given Tranz Rail permission to test its fast ferry Lynx in the Marlborough Sounds at speeds above the limit imposed one week ago.

Last Friday, a council bylaw came into force restricting the speed of fast ferries in the Sounds to 18 knots.

But yesterday the council said it had granted a Tranz Rail application to test-run the Lynx at up to 23 knots.

The trials will take place between January 5 and 7.

Council regulatory department manager Hans Versteegh denied the decision was a weakening of the council's stance on ferry speeds.

The 18-knot fixed restriction was a very black and white rule, but the dynamics of ship wake generation were more complex, so there was always an exemption provision.

"Tranz Rail have trialled their vessel between 25 and 30 knots and feel they can't comply with the wave formula in that bracket, but are of the view the Lynx will run to about 23 knots without causing a problem."

Joe Heberley, who lives in Okukari Bay at the entrance to Tory Channel, is angry Tranz Rail has left it this late to seek an exemption to the bylaw.

"They had plenty of time to do it, and now the public will be in the water and on the beaches just when the speed is increased," he said.

Guardians of the Sounds spokesman Peter Beech said the trials would meaningless if they were conducted without the Lynx carrying commercial vehicles. The ship is not due to carry commercial vehicles until early in the New Year.

He said that at 18 knots, the Lynx did not create a pressure wave. But he was concerned about the height of the stern wash when the vessel was fully loaded.