Grace Burnett opened the scoring after 16 minutes for Otumoetai with a field goal before Siennah Cowles and Sophie Gordon scored from penalty corners to have Otumoetai well ahead after 37 minutes.
But then Hillcrest fought back to ensure a tense ending to the match. They scored two quick goals in four minutes, through a field goal to Acacia Kelsen before Deborah Yung narrowed the deficit to 2-3 from a penalty corner.
The final 13 minutes were hectic with Otumoetai holding the slim one goal advantage they defended through to the final siren.
Otumoetai will contest today's final against St Matthew's Collegiate from 1.20pm.
Bethlehem College will also take plenty out of their performances this week after being the third-ranked local team going into the tournament.
They found the competition intense in the tougher Federation Cup, with Palmerston North Girls' proving too good in winning 4-0 yesterday.
But Bethlehem did have two good wins over Tauranga Girls' (1-0) and Gisborne Girls' (3-1) to finish second in their pool and were the sole Western Bay school to qualify for Federation Cup play.
Bethlehem will contest the play-off for 15th-16th against Napier Girls' at 9.50am today.