"It means that the question hasn't even been checked by someone who's competent to act as a moderator," said Mr Gibbs.
He said the blunder would "absolutely" impact on his students' grades.
Mt Albert Grammar student Zinzan Zhao, whom Mr Gibbs described as one of his top students, said he was stunned when he opened the exam paper.
"If you didn't understand something in the question then you couldn't really answer it," Zinzan said.
NZQA deputy chief executive Richard Thornton said the authority was investigating how the mistake occurred.
"That shouldn't happen and it's something we are taking very seriously."
Mr Thornton said a team had started investigating the error yesterday after four people, including some teachers, contacted the authority.
He said the marking process would make sure that no student was disadvantaged by the mistake.