Green’s Dictionary of Slang

(Aus.) a priest.

1967		‘Whisper All Aussie Dict.’ in Kings Cross Whisper (Sydney) xxxii 6/1: bag of yeast: Priest.
1971		F.J. Hardy Outcasts of Foolgarah n.p.: Yes. and the bag of yeast, old Father Flynn, thought the Red Dean, but anxious to avoid sectarianism at all times he set the religious issue to one side.
1990		Tupper & Wortley Aus. Prison Sl. Gloss. 🌐 Bag of yeast. Rhyming slang for a priest. Sometimes just ‘yeast’. Becoming dated.
1990		F.J. Hardy Retreat Australia Fair n.p.: Or if a young Ralston ruckman had been the recipient of his last thoughts, or the bag of yeast who had give him extreme unction only a few hours before?
2002		B. Kirkpatrick Wicked Cockney Rhy. Sl.