The proportion of employees who were trade union members in their main job has been steady at 18 per cent for the last three years, according to the Australian Bureau of Statistics (ABS).

There were 1.8 million people in August 2012 who were trade union members in their main job.

ABS Director of Labour Force, Cassandra Gligora, said the Employee Earnings, Benefits and Trade Union Membership survey collected the distribution of weekly earnings of employees, their entitlement to paid leave, superannuation contributions made by employers and trade union membership.

"The survey shows a higher proportion of public sector employees were trade union members in their main job (43 per cent), compared to private sector employees (13 per cent), and full-time employees (20 per cent) were more likely than part-time employees (14 per cent) to be trade union members," Ms Gligora said.

"Just under two thirds of trade union members had been a trade union member for five years or more and one in ten had been a member for less than a year," she said.

There were 1.3 million employees who were not currently members of a trade union, although they had been in the past. Of these, three quarters (77 per cent) had not been a trade union member for at least five years, while 5 per cent had not been a member a year or less. A further 6.8 million employees had never been a trade union member.