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The man was driving along State Highway 8 towards Cromwell when he went off the road and into the water about 5.15pm.
Members of the public who saw the accident unfold raced to the man's aid and helped him get out of the vehicle and lake.
Constable Leon Burdett said the public did a wonderful job helping the man escape before emergency services arrived.
"I'd just really like to thank the group who rushed to the man's aid and got him out of the lake, we're really grateful," he said.
The man, in his 60s, was taken to Dunstan Hospital in a serious condition.
The ute he was driving has since been removed from Lake Dunstan and an investigation into the cause of the crash is under way.