Reduction of payments stated in the contract by the employer

Dear Sir,

When I joined a company, my original contract said I was to get $17000 p.a. plus a day rate, they decided to cut my pay from $17000 to $14000 p.a. and said I’m to get 50% of my day rate. They did this without me signing anything in an agreement.

Also, the contract is confusing. It states that when I’m in the base and not offshore, I’m to receive only 9 hours pro-rata of my pay and at the same time it says I’m to receive only 50% of my day rate. Can you please advise?