BURY dropped to third bottom with two matches of the North Lancs Division One season to go after once again failing to turn possession into points.
Garstang dominated the early exchanges, putting Bury under relentless pressure before scoring a converted try in the seventh minute.
It was a big shock for the home side, who had hoped Garstang would provide the opportunity to claim the one win they need to escape relegation after losing narrowly to them in the reverse fixture.
The early try shocked Bury into a response and they finally started to create their own chances midway through the first half.
Fit-again Alex Waddicar broke clear in the 22nd minute before being hauled back. Bury then failed to take advantage of a tap penalty taken by the ever-alert number 8 Ben Holland three minutes later, knocking the ball forward as they raced clear.
Bury continued to dominate possession and territory after the break and managed to cut the arrears with a 55th-minute penalty.
At 7-3 the match was in the balance, before Garstang broke out of a scrum and ran the ball 60 metres to score under the posts for a converted try. The visitors then rubbed salt into the wounds, scoring a third try on the counter attack with the last move of the match.
Bury must now win at least one of their remaining fixtures to be sure of avoiding the drop.
They travel to Bolton on Saturday, April 14 (kick off 3pm).