Hundreds of crayfish washed up in a Waikato stream – and the local regional council is struggling to figure out why.

Local man Erin Hampson-Tindale reported seeing the crayfish dead during a visit to Oraka Stream in Putaruru about 1pm last Tuesday.

Regional council staff returned to the stream for five consecutive days to gather water samples and carry out an extensive search of the area. They also spoke with a number of businesses located upstream.

“It appears a single acute event has killed the koura,” said incident response team leader, Derek Hartley.

“However, the good news is that other aquatic life has been seemingly unaffected by it.”

The results of the water samples were analysed by scientists but there was nothing that clearly identifies a likely cause of death.

“While it's good news that there have been no further deaths of koura, we're continuing this week to eliminate potential causes as part of our ongoing investigation,” Hartley said. 

He encouraged anyone with information on the possible cause of the koura deaths to contact the council on 0800 800 401.

Hampson-Tindale told the Herald he was at the stream for a swim with his daughter, 10 and son, 13.

“I had to say ‘You can’t get in the water guys’.

“It was incredible. I compared it to the motorway in Auckland. It was a one-way exodus, everyone evacuated.

“To go down and witness that, it was hideous.”

He was at the stream for about two-and-a-half hours and the crayfish kept coming.

Hampson-Tindale had fished for trout at the stream for the last couple of years.

Several people were still swimming in the river.

“Am I going to take my kids down there? No.”