The Great White Butterfly will cost farmers nearly $9 million per year to control if it spreads from Nelson, acording to the Department of Conservation.
The Great White Butterfly was first detected in Nelson in 2010. Infestations can now be found within a 12km radius of the port.
DoC documents show if the Great White Butterfly continues to spread it will cost the dairy sector $7 million annually to spray forage brassicas.
The cost to sheep and beef farmers is calculated as $1.4 million per year and vegetable growers, $300,000.
DoC believes the butterfly could also wipe out 57 of New Zealand's 72 endemic cress species.
Green Party says DoC is doing an excellent job in Nelson, but the department needs more resources to deal with the incursion.