Formerly prepared by the now-disqualified Darren Weir, Grey Shadow was having her first start for Maher and Eustace in Saturday's Noel Rundle Handicap (1100m).
Eustace praised Allen post-race for his initiative in taking up the running.
“We planned to be midfield, or a little bit better, but Johnny was able to get there quite easily and was able to control the race,” Eustace said.
“She trialled very well at Cranbourne and her work had been very sharp at home.
“It didn't look the deepest race coming into the off-season now and she put them away quite easily.
“It will be all about getting her some black type now.”
Eustace said the stable would consider upcoming stakes races for the mare in either Adelaide or Brisbane.
“We're still getting to know her a bit,” Eustace said.
“There's a lovely program for mares in Adelaide and she definitely appreciates cut in the ground so Queensland is a possibility, so it will be about finding the right race.”