As a teenager living in Lewisham, he played a few games for London Irish youth sides. One of his team-mates was a full-back called Tony O'Malley, who spent a season playing for the Oriental Rongotai club in Wellington, the club that later nurtured the likes of Ma'a Nonu and Julian Savea. When he came back he told Gallagher that the side - made up largely of Polynesian players who liked to run with the ball at every opportunity - were short of a kicking full-back and why didn't he try for the role. There was just one slight problem: Gallagher then played as a centre. But as he was 19 and thought anything was possible, he decided to head to New Zealand anyhow.
He found he was an immediate success. So much so, that after his first three-month visa had elapsed he thought it might be worth staying on. Not least because Wellington was a beautiful place, very different to Lewisham in the mid 1980s.
"I approached the club chairman Don Bond and he said, 'Come and see me in my office and we'll see what we can do'," recalls Gallagher in a south London accent which betrays not a hint of Kiwi. "So I did and when I turned up I saw that the sign on the door read: Director of New Zealand Immigration Services. I thought: 'I might have a chance here.'?"
However, Bond handed him a sheet of paper with the list of the most-needed professions. At the top was brain surgeon, at the bottom was qualified chartered accountant. As a young man on a gap year ahead of taking up a place at the Metropolitan Police College, Gallagher didn't see how he might fit in.
"But Mr Bond said, 'Look at the bottom of the sheet," he remembers. "It said 'other'. He said, 'there you are then'. And I was in." So in was he, within a year, as the leading young points kicker in the country, he found himself chosen for the All Blacks.
"I remember when I turned up and the others heard my accent they were a little surprised. After all, this was not long after the 1983 British Lions tour that was a complete whitewash. Some them wondered what the hell an Englishman was doing there."
Despite that, and despite his inexperience when it came to the haka, he was quickly assimilated into the side.
"They were unbelievably welcoming," he recalls. "John Kirwan, David Kirk, Jock Hobbs: amazing guys. And the better the players around me, the easier I found the game. Funny that."
Within a year of his debut, he had won the World Cup. Not that it was the sort of life-changing experience it will be for those who win on Sunday morning. There was no fanfare, no open-top bus parade; then working as a policeman, two days after victory he was on the beat in Wellington. And three years after the win, while on tour with the All Blacks, he experienced a moment that really did change his life.
"We played at Twickenham, all my family and friends were there and I just felt suddenly so homesick," he recalls. "And that was it. I took up rugby league and never went back."
After a spell as a teacher - and a job this World Cup working as a radio commentator - these days he works for a property company just down the road from where he was born.
His two sons both play rugby, with 23-year-old Alex at Rosslyn Park, and Matt, 19, on Saracens' books. Family connections mean Matt, a fullback like his father, could declare for any one of New Zealand, England, Ireland or Italy.There is no doubt, though, where their dad's sympathies will lie come kick off on Sunday.
"I may be an Englishman, but there's only one team for me."
Once an All Black, always an All Black, it seems. Even when you come from Lewisham.
