Trailing 24-15 at the break, Newcastle scored four tries in 18 second-half minutes to turn the game around.
Scrum-half Takulua crossed the line three times and also added another nine points with his boot off the tee.
Northampton, who earned a bonus point of their own with four tries, was undone by a series of defensive and handling errors in the second half.
A second successive home win for Newcastle lifts them above Northampton to seventh and just a point behind sixth-placed Harlequins.
