Girls as young as 11 have reported being harassed on Sydney streets, a report by Plan International Australia which surveyed 500 women aged 18 to 25, has found.

The survey, facilitated by Ispos Australia, found 90 per cent of women didn't feel safe in Sydney after dark with many complaining of being harassed, followed, cat-called, groped or leered at.

Of the young women surveyed who have experienced street harassment in the city, more than a third were between 11 and 15 the first time it happened, the report, published on Tuesday found.

Milly, who is 16 now, says the first time she was cat-called she was in Year 4 walking home after ballet practice.

"A car with P-plates slowed down next to me. In it was a group of men who started honking, wolf-whistling and shouting comments I didn't understand," she said in the report.

"I was alone and terrified and ran the rest of the way home."

The 16-year-old says she's still scared to walk alone and speeds up when she hears footsteps behind her and makes a mental plan of what to do when someone approaches her.

Just one in 10 young Sydney women reported feeling safe to go out at night while nine out of 10 felt afraid or intimidated in the city after dark, the report said.

The development and humanitarian organisation found street harassment in Sydney wasn't limited to the night with 43 per cent of respondents who had experienced harassment saying it was more common during the day.

Plan International Australia's chief executive Susanne Legena said street harassment was common and often silently endured.

"What this report tells us, loud and clear, is that cat-calling and menacing behaviour is not 'harmless fun' or 'a compliment'. It has real and lasting repercussions," Ms Legena said in a statement.

"We know that Sydney is a fantastic city and one of the safest in the world but even so, it could be improved by acknowledging street harassment is a problem that really needs to be addressed."

Almost half the women surveyed felt unsafe using Sydney's public transport alone, with the report saying women have had men expose themselves on quiet trains, whisper inappropriate comments in their ears and have been groped and brushed up against inappropriately during peak hour.

In almost all cases of harassment reported in the survey, the perpetrators were men.

Nearly all the young women surveyed said they would feel safer if Sydney was designed with women's safety and wellbeing in mind, councils took women's safety more seriously and public transport authorities did more to make trains and buses safer.