The proportion directorships held by women has doubled since 2008 but is still below the level at which the Human Rights Commission (HRC) says a woman's opinion can make a real difference.
A survey by consulting firm Strategic Pay shows women directors increased from 13% of the total to 24% in the past six years.
However, the HRC says governance experts regard 30% as the minimum level at which women's perspectives can have a real impact.
Strategic Pay chief executive John McGill said there was social pressure on companies to increase the gender balance on boards.
"There are a lot of women that are well qualified out there that can take up these positions. You might just have to look a little bit harder for them," he said.
"But certainly the capability and the competence of the ladies that I work with in my two boards are just as high as anybody."
Mr McGill said men and women directors were paid the same.
