The duty, dubbed a "snooper's charter", forces all teachers to co-operate with headteachers or assessors dealing with applications for the pound;2,000 threshold pay rise.
General secretary Doug Mc-Avoy said the duty would create an atmosphere of suspicion in schools and destroy teamwork and co-operation. The vote gives local members support in taking industrial action up to and including strike action where attempts are made to require the use of a teacher as an "informer".
During the debate, delegates were told that colleagues would be expected to spy on one another, pitting teacher against teacher in a squalid scramble for cash.
The Government said the duty refers to line manager, for example head of department, of the teacher attempting to cross the threshold. The School Teachers Review Body sanctioned the amendment to the teachers' pay and conditions document, bringing about the change.
Conference called for the abolition of the review body, to be replaced by collective bargaining.
Mr McAvoy arrivedat conference with some good news for his delegates. The union had won leave by the High Court to challenge whether the government should have allowed consultation on the new duty.
Justice Moses described the Government's proposals as "dispiriting" and questioned when teachers would find time to assess their colleagues. - after hours, over the weekend or "in the middle of Latin lessons?"
He found the Government had a case to answer over consultation, whether the correct piece of legislation was used to add the duty and the rationality of a law which makes a requirement on all teachers when it will only be applied to certain members of staff.
It could also mean that Wales, which has rejected the assessment by results element of the pay plan under its 1986 powers, becomes a performance-by results free zone.
The Department for Education and Employment accused the NUT of consistently misleading teachers about the practical effects of the pay changes.
A spokesman said: "The judge has not heard the department's evidence and he will do so when the review takes place. When it does, we are confident that our position will be vindicated."