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The 13-year-old twins were fishing off the wharf near the Port of Otago building in Wharf St about 5.30am when a young woman got into difficulty in the basin.
Jack said he and his brother went fishing there most mornings, and had seen people jump in, in the nude.
"But they were just fooling around. This lady looked like she needed help.
"I was thinking one of us had to jump in and save her."
"I wondered what the hell he was doing. Then he just jumped in and swam over to this lady."
Jack said when he reached the woman, he tried to hold her head above the water while his father, Keith, and a friend called Dunedin police.
"She seemed grateful."
Police arrived soon after and Constable Charlotte Davies also jumped into the basin to help the woman back on to the wharf.
"Anyone who is prepared to jump into the harbour at that time of morning is fantastic."
However, he was concerned about the lack of exit points for anyone falling into the basin.
"There's a lot of people fishing down there now. People are going to end up in the water sooner or later and we have had difficulty getting people out of there in the past."
Port Otago revised safety features around its wharves following a recommendation from Southern region coroner David Crerar in relation to Dunedin man Regan McCormack (20), who drowned below Fryatt St wharf last July.
Port Otago chief executive Geoff Plunket said, in response to the review, two new ladders were placed in the basin area - one each on the Fryatt St and Birch St wharves.
They did not put ladders on the Wharf St wharf because as a public reserve it was the responsibility of the Dunedin City Council.
Council parks and reserves team leader Martin Thompson said he would investigate the issue.
Jack said the best part of his day was catching two big fish.
The fish David was reeling in at the time of the incident turned out to be a 5kg salmon - his first for the season.
After the rescue, not to be outdone, Jack caught his own fish - a 5.5kg salmon - both of which made for a satisfying lunch yesterday.