It was sentenced by Judge Brian Dwyer in the Auckland District Court on Friday.
MMA is a colourless, volatile liquid used to make resins and plastics, described as having a pungent or fruity odour.
It leaked from a newly repaired storage tank. Allnex had received advice to replace the tank but had also been told that it was repairable.
Judge Dwyer said he was prepared to give the company the benefit of the doubt, in that it had honestly relied on the advice that the tank was repairable.
However, he said the company had knowingly taken a risk in repairing the tank, and that an earlier spill should have alerted it to a problem involving a pipe which connected the containment bund to the stormwater network.
Kerri Fergusson, the Auckland Council's Manager of Compliance Response and Investigations, said the council was happy with the outcome of the court case.
"This sends a strong message to those businesses who handle contaminants that may be harmful to our environment, to ensure their processes are sound to prevent anything happening like this in future," Fergusson said.
"The death and destruction of the freshwater wildlife and fauna was extremely sad to many involved, especially mana whenua and the community, and this successful prosecution of those responsible should act as a deterrent to others."
The company spent about $400,000 on a clean-up of the inner reaches of the Manukau Harbour which took several weeks.
However, Judge Dwyer gave the company no credit for this, saying it had done no more than was required to deal with the spill.
Staff at the Allnex site in Penrose said the only person who could comment on behalf of the company was travelling on Friday and was unavailable.
