Union membership has declined to 1.7 million people, or about 19 per cent of the workforce, data released by the Australian Bureau of Statistics has found.
This was a fall of 89,600 from the previous year.
The proportion of employees who were trade union members also decreased from 20 per cent one year earlier.
The ABS found that union membership was stronger among full-time employees (21 per cent) than part-time employees (14 per cent).
Union membership was particularly strong among public sector employees, where 41 per cent were in a union, compared with those in the private sector (14 per cent).
The union membership data continues a long-term trend.
In 1993, 2.4 million people, or 38 per cent of the total workforce, were union members.
The number of union members had been steady for most of this decade at about 1.9 million people, but the proportion covered by unions was in constant decline as new entrants to the workforce failed to join a union.
The data released this week by the ABS showed that in August 2007, there were nine million employees in Australia.
Of these, 53 per cent were men, of whom 86 per cent were full-time workers.
Women made up 46 per cent of employees, and a much lower 56 per cent of these were full-time workers.
Employees earned, on average, $916 a week in their main job.
This ranged from $1,216/week for full-time employees to $412/week for part-time employees.
Male full-time employees earned, on average, $1,212/week in their main job, compared with $971/week for female full-time employees.
The ACT had the highest mean weekly earnings ($1,122), while Tasmania had the lowest ($773).
The mining industry had the highest mean weekly earnings for full-time employees ($1,772), while accommodation and food services had the lowest ($781).
The age group with the highest mean weekly earnings for full-time employees was 35-44 year olds ($1,284), while the lowest was 15-19 year olds ($493).
Three-quarters of employees were entitled to paid holiday leave and/or paid sick leave in their main job.
For full-time employees, the proportion entitled to these conditions was 89 per cent.
