Casual workers have won the right to demand a permanent full-time or part-time job after 12 months under a landmark Fair Work Commission ruling.

But employers will still have the right to refuse the request if the change would substantially alter the worker's hours to accommodate them as a permanent staff member.

Unions had called for the right to be available after just six months and for the minimum number of daily hours worked to increase to four, but these requests were rejected.

In its decision on Wednesday, the Fair Work Commission said it was necessary for modern awards to contain a provision allowing casual employees to ask for conversion to permanent full-time or part-time work after 12 months.