Tourists are flocking to a popular Canterbury river to catch a rare glimpse of hundreds of dancing sockeye salmon bursting out of the water as spawning season gets under way.

Groups gathered on a Twizel bridge to spot the southern hemisphere's only population of the fish, who were thought be extinct, scramble up the riverbed in search of the perfect spot to lay their eggs.

It comes after thousands of the fish were spotted for the first time in decades near Lake Pukaki, in the Mackenzie District's alpine rivers.

SUPPLIED The fish was thought to be extinct in the late 1980s. Reported sightings started filtering in about 2005, but none were confirmed until this season.

The fish was though to have become extinct in the 1980s, but reported sightings started filtering in about 2005. This season marked the first time a sighting had been officially confirmed.

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Central South Island Fish and Game officer Jayde Couper said there was about 1000 spawning fish in just one of the lake's tributary streams this year. The sockeye's comeback from the verge of extinction was "widespread".

"The fact sockeye appear to have come back from the dead is heartening and a positive sign of the health of the fishery in the Waitaki Lakes," Couper said.

The fish, who live in lakes and were only seen when they returned to rivers to lay eggs, had even turned up in areas they were not known to exist in previously.

Sockeye had also been spotted in almost all of the rivers and streams flowing into Lake Benmore, and in the Lower Ohau River, the Twizel and Fraser Rivers and the Tekapo River.

Couper said the State Highway 8 bridge, near Twizel, was a popular spot because people could watch the fish from the platform without disturbing them.

He warned people not to interrupt the fish, who are protected under the Conservation Act.

"You can't catch, net or spear the fish, or even walk in the river bed and trample their nests."

Fish and Game was developing a method of monitoring the growth of the species it had thought to be extinct.

"There are too many salmon and not enough time and resources to count them all," Couper said.