A man has been seriously injured after two jet-skis collided at Sandridge Beach at Port Melbourne.

A 50-year-old rider was injured after his jet-ski was hit by the second jet-ski, a Victoria Police spokeswoman told Fairfax Media.

A man who crashed into a house near Geelong is being flown to The Alfred hospital in a critical condition.

The 19-year-old man riding the other jet-ski was being interviewed by water police, but no charges have been laid.

Ambulance Victoria spokesman John Mullen said they were called to help the man just after 1pm.