It depends what the company does upon your refusal. If it went ahead with a unilateral reduction in your pay, you would be entitled to do one of the following:
A] Resign, and make a claim for constructive dismissal to the EAT. You would be required at least to 1. utilise the company grievance procedure and 2. notify the company that you were working under protest.
B] Make a claim to a Rights Commisioner for Illegal Deduction of Wages, per the Payment of Wages Act.
C] Make a claim under common law for breach of contract. Again, as in A] You would be required at least to 1. utilise the company grievance procedure and 2. notify the company that you were working under protest.