Marine scientist Associate Prof Christopher Hepburn, of the University of Otago's marine science department, said the bright-green carpet was made up of ulva, a type of edible green algae which was widely distributed along the coasts of the world's oceans.
He was unsure whether the sea lettuce was the Ulva compressa or Ulva intestinalis species.
The Andersons Bay infestation probably resulted from the rotting of a huge blanket of white krill which invaded the inlet
late last month, Prof Hepburn said yesterday.
As the krill rotted, nutrients would have been released into the water.
Those nutrients combined with the hot sun and warm water temperatures and a fairly static body of water to create ideal conditions for the sea lettuce to flourish.
Although he has lived locally for the past eight years, it was the first time he had seen such a plant infestation of the inlet, Prof Hepburn said.
``But it is quite common elsewhere when the conditions are right.''
The bloom itself was not toxic to humans or animals, and fish could eat the plant.
Humans could also eat it, ``although I wouldn't recommend eating the sea lettuce from here'', Prof Hepburn said.
And he thought there could be problems if the plant mass rotted too quickly. Low oxygen conditions would be created and it could become ``rather smelly''.
``But we'll have to wait and see.''
In the meantime, it was important people understood that while the mass was a form of algal bloom, it was not toxic.