Two Reihana penalties and a Soane Tonga'uiha try put Northampton further ahead before the Chiefs rallied.
Tonga took a 12-10 lead into the break, but the drama came in the second half.
The two sides of Tonga's brilliant stand-off Feleti Mateo were in evidence soon after.
Tonga has a particular problem among its female population with seven in 10 women obese.
Tonga coach Quddus Fielea said South Africa had underestimated them by fielding a second-string side.
Tonga captain Nili Latu hailed the team's improved discipline and confidence in their own game.
The extraordinary scene was captured along the Tonga Trench during a research expedition last summer.
Tonga's prime minister, Lord Tu'ivakano, says the courts will impartially consider a request for extradition.
The dead included 22 in American Samoa, 82 in Samoa and seven in Tonga.
Both should be fit for Saturday's game against Samoa, while Jason Robinson hopes to return against Tonga.
Boxers from Tonga are training in Liverpool as they prepare to take part in Olympic Games qualifiers.
His team-mates Phil Dowson and Soane Tonga'uiha are contenders, with Sarries duo Schalk Brits and Ernst Joubert.
This will assist to further build and connect the many on-going ESD activities already underway in Tonga.
The trench is a highly active fault line running north from New Zealand towards Tonga and Samoa.
The demoralised Dragons let Catt through for a walk-in try after a neat off-load from Tonga Lea'aetoa.
Those 12 are Argentina, Australia, England, Fiji, France, Ireland, Italy, New Zealand, Scotland, South Africa, Tonga and Wales.
The man also hurt his back when the 11.6m (38ft) vessel rolled after leaving Tonga two days ago.
His Digicel Group now operates in dozens of Caribbean islands, Honduras, Haiti, Fiji, Tonga and Papua New Guinea.
Billy Slater, Brent Tate, Cooper Cronk, Willie Tonga and Brett Morris all scored in a one-sided first half.
The 22-year-old will tour Fiji, Tonga and Samoa for three and a half weeks starting on 8 June.
Hull star Radford was part of the Federation Shield-winning England team that overcame a very physical Tonga side.
But Tonga'uiha had made it clear that he wanted to remain at Northampton.
Hola then added another three points before Tonga took the lead when Taione finished off a powerful forward drive.
Tonga, ranked four places below Samoa, reduced the deficit through a second Hola penalty as the half ended 12-6.
Between June and November, humpback whales congregate in Tonga to mate and breed.
But Wales were beaten 19-10 in the semi-finals by Canada, who lost to Tonga 14-12 in the Plate final.
On Tuesday, a magnitude 8.0 quake-triggered tsunami killed at least 111 people in the Samoan islands and Tonga.
The Fiji, Samoa and Tonga select lost 38-20 to Wales at the weekend.
Tonga was another step forward and at times we started to look quite dangerous and creative as an attacking force.
Saracens had felt aggrieved over Tonga'uiha's U-turn and had sought to persuade the 28-year-old to honour his agreement with them.
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