NSW teachers and politicians have lashed out at the potential conflict of interest that a multi-billion dollar British private education firm has in the state's education system.

Pearson, the world's largest education business has held the $41.6 million contract for marking NAPLAN in NSW since 2011. The $10 billion global business generates millions of dollars out of text-book sales every year.

Dr John Kaye, left, at NSW Parliament house. Credit:Kate Geraghty

At the state's budget estimates last week, Greens MP John Kaye said that the conflict of interest between holding a contract that allows Pearson to mark NAPLAN tests and generate revenue out of the same exams was obvious.

"This is a live issue in NSW. It is critical that the government erects barriers in education to the interests of private companies," he said.