At the start of their careers, men and women earn almost the same, but a new report reveals the gender pay gap widens for women with every birthday.

By the time women reach their 30s, their earnings flatten and men's climb, with female bosses earning almost a full median salary less than male managers on average in Australia.

A Workplace Gender Equality Agency (WGEA) report has found the gender pay gap sits at 1 per cent in favour of men for workers in their 20s.

But the gap that started off as a hairline crack for women in their 20s widens in their 50s, with female bosses earning $85,600 less than male managers, on average.

For non-manager roles, men in their 50s earn 31 per cent more than women their age -- a $52,000 difference on average.

The report used information reported to W

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