Yoshiko Cowell watched in horror as her daughter, Kotomiyo, tipped over in her pram before rolling down a sea wall into the water at Portobello at 11.30am.
Trapped under the pram, the 3-year-old managed to keep her head out of the knee-high water and scream for her mother, who "jumped in without thinking".
She was able to grab her daughter and the pram before it could float away.
Seeing their plight, a motorist stopped to assist. He helped lift the pair - and the pram - out of the water and, as he was offering them a ride home, his car rolled into the harbour, just metres away.
"He just laughed ... I was shocked," Ms Cowell said.
The man had stopped after he saw the pram roll down the 2m-high wall into the water. He turned on his car's hazard lights, then rushed to the aid of the stricken pair.
Ms Cowell said she planned to meet him to thank him for his assistance.
"He did such a good deed and then that happened to him ... It is just not fair," she said.
"I wish I could afford to buy him a car."
Attempts to contact the 59-year-old man at his Harwood home last night were unsuccessful.
Senior Constable Lox Kellas, of Portobello, praised the man, who was a "Good Samaritan and responded very quickly".
Unfortunately, the man had failed to apply his handbrake "in the heat of the moment".
The car was later recovered.
Ms Cowell said the pair had escaped relatively unscathed, apart from a waterlogged cellphone and a few grazes on her daughter, but they were both in shock after the incident.
"I got home and burst into tears ... It was all just so bizarre." Although the brake on the three-wheeled pram had been applied, that had not prevented it from tipping over and it served as a reminder to parents to be careful near water, she said.
