The Council of Trade Unions says farm workers need independent representation to ensure they have employment contracts and are properly paid for the hours they work.
Latest figures from Federated Farmers show while the number of farms with written contracts grew by 9% since last year, fewer than a quarter have a contract with their employees and only 40% keep records of the hours their employees work.
The data was collected as part of an annual farm employee remuneration report that the federation conducts with Rabobank.
President Don Nicolson says farmers who do not keep good records and have written contracts with their employees, are putting themselves at risk.
Mr Nicolson says employment disputes are one of the biggest problems that the federation is asked for advice on.
He says either farmers are not aware of the law or they are just being remiss, as employees have the right to ask for an employment contract.
Mr Nicolson says the federation will be holding seminars throughout the country to educate its members on employment-related matters.
But the Council of Trade Unions says there is a need for a union.
Secretary Peter Conway, says farm workers are often in isolated locations where it's difficult for them to get advice, so they need a union.