Rogue jetboaters have been roaring up small West Coast creeks, infuriating farmers and anglers and possibly leading to the death of a horse.
As many as 12 boats from the Coast and away have been spotted tearing up the La Fontaine Creek, at Hari Hari, and the Poerua River, near Lake Brunner.
"We suspect these are the same group of individuals," Fish and Game West Coast manager Dean Kelly said today.
In a joint statement, Jetboating New Zealand and Fish and Game warned that the drivers could face prosecution.
Hari Hari farmer Mary Molloy was at home about a month ago when "all hell broke loose" on the La Fontaine, and about 12 jetboats flew past.
About an hour later, they roared back down again, sounding like a "taniwha".
"I was waving at them, and they were waving back," Mrs Molloy said.
Three of her horses in a paddock took off, and one an old breeding mare died soon afterwards.
"There was so much noise a staff member would not milk, because he had previously had the cows (jumping) over the top of him."
At a neighbouring farm, 30 calves took off and took three or four hours to return.
Mrs Molloy said the banks of the creek were like "sugar" and left crumbling.
She said the jetboaters had come from nearby Lake Ianthe. Previously, jetskis had roared up the even narrower Thorpes Creek.
The waterways were just too small for jetboats, she said.
Fish and Game and Jetboating NZ said they were worried about the safety of anglers and other recreational users with the high-speed boating.
The boaties had been "causing havoc" in small tributary streams that were otherwise closed to any river craft travelling greater than 5 knots.
Mr Kelly said anglers who had waded into the streams would have neither room nor time to get out of the way.
"It's an accident waiting to happen."
To compound matters, it had been happening during the sports fish spawning season, when eggs and fry were present in the gravels of the streams.
Jetboating NZ spokesman Paul Mullan said they had worked hard over many decades to advise jetboaters of which rivers they could boat legally.
But some rivers and streams were off limits for a variety of reasons, including spawning.
"We do not condone any jetboater, whether a member or not, breaching these regulations. Offenders are liable for prosecution under the Conservation Act for this disturbance to freshwater fish spawning," Mr Mullan said.
Mr Kelly said it had got to the stage where landowners adjacent to the waterways who had previously granted access were now reluctant to let people through, because it could be unsafe.
The speed limit is regulated by Maritime New Zealand. All freshwaters have a speed restriction of 5 knots within 200m of the shore and the only areas where higher speed is permitted is where a speed uplifting has been granted either permanently or temporarily.