As she was about to pass it to force the driver to stop, the dog's collar broke. She could not avoid hitting the animal, which may have been a bearded huntaway.
"Thankfully, it was killed instantly," she said.
The tractor driver and Mrs Crosby consoled each other where the dog died but the ute kept going.
"The driver didn't stop at all. They don't know how their carelessness caused the horrible death of their dog," she said.
"It was a sick way for a dog to die. It was just senseless."
Mrs Crosby alerted the SPCA.
Masterton animal welfare officer Lesley Gibson said that when she arrived at the site yesterday, the dog was nowhere to be seen.
However, workers at the sewage ponds said it had been there when they arrived that morning.
"We want to find out who the driver was. It was a horrendous situation," Mrs Gibson said.
She said Mrs Crosby did everything she could.
"It was actually a blessing in disguise that she hit the dog.
"If dogs are going to be carried on the back of utes they need to be tied on in such a way that they can't fall off the edge."
Anyone with information about the dog or the ute driver should contact Masterton SPCA on (06) 377-1912.