The Professional Firefighters Union says some of its members are being intimidated for sending out more fire engines to emergencies than the rules require.
Since the end of April, the Fire Service has limited the number of engines responding to fires at buildings fitted with sprinklers to two.
The union claims the Fire Service has not properly assessed every building for sprinklers and says some should get more than a two-engine response.
It says some staff have been threatened with disciplinary action if they continue to breach the policy.
The Fire Service says there has been no intimidation but says discussions have taken place with some employees.
It says the two-engine policy frees up resources for other emergencies.