At around midday today (June 26), a dog being walked by a professional dog walker swam across a stream and dragged the two sheep into the water, drowning them.
Owner Jenny Flake said the animal bit and bothered others in the flock, but it was too soon to be tell how many had been attacked.
Attacks on sheep are on the rise, with Jenny estimating her flocks of around 2,000 animals are being bothered once a week.
"To drag them into the river and drown them is just awful, what a way for them to go," she said.
"These dog attacks are getting worse. More and more people are using professional dog walkers - paying people to walk their dogs responsibly, and they're not.
"The whole thing is on the increase and that's only the ones we know about. Sometimes we will get the sheep in and we will see they've been bitten or attacked and we don't know when or how it happened."
Jenny said it was up to dog walkers to take responsibility for their animals and urged them to keep their dogs on the lead around animals.
The dog walker from today's attack called the police, she said, but had protested she was not aware the dog would be able to swim across the stream.
Dog owners had a responsibility to make sure their dog walkers are responsible, said Jenny, to expect the unexpected on a walk and to have insurance for their animals.