Police are investigating after three women were sexually assaulted while walking home in North Dunedin in separate incidents at the weekend.

Inspector Wil Black, of Dunedin, said the three women were assaulted either late at night or in the early hours of the morning over the weekend while walking home.

Every year when students returned to town police recorded an increase in such incidents, he said.

The Criminal Investigation Branch was now actively investigating the attacks and speaking to people of interest.

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Black urged people to be vigilant in the wake of the attacks.

"Just look after yourself, look after your mates.''

Manwhile, students living on a notorious party street are calling on troublemakers to stay away after four students were seriously assaulted by non-students.

Police attended four separate assaults, in Hyde, Dundas and Castle Sts - including one in which a male student suffered a broken nose after being hit in the head with a bottle - early on Sunday.

Another male student was admitted to hospital with a broken jaw, after a king hit.

University of Otago proctor Dave Scott said yesterday the university was ''extremely concerned'' about non-students causing trouble in the student area.

Students in Hyde St yesterday said it was common for non-students, including patched gang members, to come into the street looking for fights.

Alex Collier said this week someone in the street had been stabbed with a screwdriver and another person had been beaten up.

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''It's always non-students because they can just come here and do what they want without repercussions,'' he said.

The problem would be difficult to stop and troublemakers would continue to be drawn to the street as long as there were ''still students here''.