Both teams were pressing hard early in the match, going goal for goal as the Saints held a small lead of 3-2 at quarter time.
Both teams had equal opportunities at goals during the third term but it was the Dolphins accuracy that gave them an 11-7 advantage at three quarter time.
The final term belonged to Dolphins, holding the Saints to just one goal.
Dolphins will now meet City Beach, who went down to Melville Sharks 16-9.
The preliminary final will be played at Challenge Stadium on Tuesday at 7.15pm.