Two parents lost at sea off Perth’s coast late on Wednesday had only intended to take their new jet ski for a quick spin, before finding trouble.

The pair was plucked from their machine after their kids, who had waited anxiously for hours for them to return, sounded the alarm.

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A 41-year-old woman and a 37-year-old man had hit the water at Warnbro Sound in Port Kennedy about 8pm and weren’t rescued until about midnight.

By then they had drifted 5km out to sea.

Their two distressed children, aged five and 11, were found by a stranger waiting at a boat ramp about 11pm.

The stranger contacted police and a sea search involving Water Police was triggered immediately.

Crews on a police helicopter spotted the pair on their broken down jet ski floating off the coast about 11.40pm and a Water Police vessel was able to pull them to safety 20 minutes later.

Senior sergeant Grant Rosman said it was “very fortunate” authorities were alerted.

“However three or four hours out to sea is still a long time to be out there,” Rosman said.

The jet ski was a recent purchase the couple had hoped to take for a quick joyride.

But they were in trouble when it broke down and there was nothing onboard to help in an emergency.

Rosman said basic safety gear including life jackets, an EPIRB, flares and navigation lights would have allowed them to call for help quickly.

The jet ski was towed to shore, where the children were reunited with their parents.

Neither of those rescued required medical assistance.