A Whanganui grandmother wants to thank the mystery Samaritan who pulled her grandson from the Whanganui River.

Jennifer Grammaticogiannis' Saturday stroll with her daughter Alexia and grandchildren Theo, four, and Benjamin, two, turned into a high drama around 9.30am when Theo rode his bike off the track and fell into the river.

They were walking along the new public walkway along Taupo Quay, slightly downriver from Trafalgar Square.

"All we could see was his helmet on the surface of the water, it was terrifying," Ms Grammaticogiannis said.

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"Alexia dived straight off the bank, about a 2.5 metres drop, and broke her foot. She was hanging onto Theo, I was trying to console Benjamin, and Alexia needed help - it was horrible."

A passing walker stepped in to save the day. She scrambled down the bank and pulled Theo from the water and helped him back onto the walkway.

"She was wonderful and I really want to thank her but I didn't get a chance. I hope she reads the story and accepts our thanks, we are so grateful."

The boys were upset but otherwise unharmed.

Not so lucky was their mum, who received two fractures and now has her foot in plaster.

The good Samaritan can contact Ms Grammaticogiannis through the Chronicle.