"It's a lot bigger than all of us realised, to be fair."
The government has allocated $85m to deal with the outbreak, but a looming report will likely spell out a "significant" bill, Finance Minister Grant Robertson said earlier.
The Budget added another $9.3m of operating funding to improve overseas biosecurity systems, and Robertson said the Crown needed to "look at different ways of dealing with biosecurity incursions" which have been on the rise in recent years.
Commenting on other aspects of the Budget, Lewis said the rural sector suffered from underinvestment in services and it appeared it was dominated by "the big three" - education, health and social welfare.
The primary sector also got a $15m boost to the Sustainable Farming Fund and a $5m increase for the Overseer farm management tool, both of which aim to lift environmental practices on farms.
The new policy direction also stripped out and reprioritised $163m over the four-year period, cancelling subsidies for irrigation and the primary growth partnership (PGP), and dropping the food export value and trusted trader programmes.
Lewis said the PGP would be missed because of its contribution to research and development in the rural sector.
He welcomed some elements of the Budget but overall felt that it largely ignored the needs of rural New Zealand.
"I see that there will be big budget spending on education and health, but in the rural areas we are struggling just as much as the urban areas," he said.
"There are lots of little things in there that are good," he said. "In terms of the primary industries, there were some crumbs."
Overall he said it was a "miss" for the rural sector.