A Hamilton man was threatened and robbed after he used social media to solicit sex.

Police said an investigation has been launched, after a man attempted to solicit sex using social media last Monday.

Acting Detective Sergeant Andrew Hawke said after making contact with a woman, the man met with her in the Hillcrest area of Hamilton.

Two men and a woman also joined the woman.

Mr Hawke said the four offenders then threatened the man, blackmailing him to hand over money.

He said they threatened him and his flatmate with a knife before entering their address and robbing him of money.

Police believe other people may have been victimised by the group.