Eyewitnesses told the Samoa Observer that the group was having a casual swim and that the surge of water appeared out of nowhere.
A boy who had been sitting near the river bank told the newspaper he was shocked at what he saw.
"I thought it was a doll ... it wasn't until I walked closer to the rocks that I saw it was a person.''
The boy called to another member of the public for help. They managed to bring the person - a young woman they thought was in her late teens - in.
They tried to help her, but it was too late.
The man who helped bring the young woman's body in told the newspaper: "She was already dead when I got to her. I called out to my friend to call the police.
"She looked young - about 19. It was so sad.''
Four other family members were rushed to Upolu island's main hospital Moto'otua Hospital in Apia. They remained there late tonight, where they were being treated and comforted by extended family. Meanwhile, patrol boats were scouring the murky river for signs of the missing person.
Police would not to comment when contacted by the Herald tonight, saying officers were still at the scene carrying out their investigations.
Other officers were at the hospital, understood to be questioning members of the family.
A doctor at the hospital confirmed the family were holidaying from New Zealand.
"The family's from New Zealand - they're here for vacation. They are Samoan but they don't live here.''
The Vaisigano River is notorious for its unpredictable nature.
Running through Apia - and just past famous Aggie Greys resort on the capital's main street - it can some days be completely dry and at other times it can cause flash floods which spill on the city's main road.
