A Kiwi toddler has been saved by a "heroic" passer-by after falling into a treacherous Gold Coast river.

Three-year-old Lucas Wallace was walking across a bridge in Surfers Paradise with family members when he fell into the Nerang River just before 7pm on Friday.

His mother Jackie watched in horror as Lucas was swept under the Chevron Island Bridge by the strong current.

Tahitian student Charles Tetaahi, 35, witnessed the fall and dived into the water after Lucas.

Jackie Wallace said she had feared the worst as she watched her son go over the side.

She told 3 News: "I'm just so grateful that they got him out, really grateful that they've helped. I am still trying to get over it."

Lucas' uncle Philip Anderson said they had been walking across the bridge and "one second later he was falling through a gap between the footpath and the bridge".

He said the accident should serve as a warning to other parents. "I guess it's just a sign for people to be careful. We are just really glad that we have got him back."

Police praised Tetaahi's selfless actions and said his quick thinking had saved the boy's life.

Tetaahi said he grabbed hold of the boy and they were swept 50m downstream before making it to shore.

"I saw this baby pass, he had his hand (out) ... I just jumped. I swam with him, I grabbed him, and I said to him: 'I'm here, I'm here', that's it. I didn't have a choice, I just jumped."

Lucas was taken to the Gold Coast Hospital and kept overnight for observation.

The accident happened on the bridge linking downtown Surfers Paradise with Chevron Island, which lies just inland.

In the year to June 30, 2009, 302 people drowned in Australia.