Three missing in boat off SA coast

A search is continuing off South Australia's southeast coast for a missing boat with three people on board.

The seven-metre boat failed to return to the Cape Jaffa boat ramp on Monday night, police say.

It had headed out to sea around lunchtime on Monday and had been due to return in the evening.

Locals spotted an empty trailer at the ramp on Tuesday morning and raised the alarm.

Police boats, State Emergency Service volunteers and local fishermen are involved in the search along with the police helicopter and a fixed-wing aircraft.