Organisations in New Zealand have long been aware of a gender pay gap but haven't had the statistics to back it up - until now.

LISTEN ABOVE: Larry Williams talks to the president of the CTU, Richard Wagstaff, about the jump start for pay equity.

Statistics New Zealand, in collaboration with the Ministry for Women, has completed research showing mothers earn on average five dollars less an hour than fathers, a difference of 17 percent.

Statistics analyst Andrea Fromm said this is what is known as the motherhood penalty, an area not well researched in New Zealand, about the consequences of women having children.

Ms Fromm said the motherhood penalty becomes even clearer when you compare parents income to earners without dependent children.

She said the pay gap between women and men without children is only five percent.

"Mothers with dependent children earn more than $23 an hour, on average, almost $1 an hour more than men and $2 more than women without children," insists Statistics New Zealand manager Stephen Oakley.

Research shows that mothers may get less because they spend more time caring for children, which interrupts full-time paid work and which may reflect breaks from work and reduced work experience.