The Magpies could not hold out an impressive Eels outfit that will now face South Sydney in the decider next weekend.
Despite finishing the regular season in fifth place, the Eels came out firing and went into the half time break with a 32-0 lead.
Eels centre Mark Griffiths ran in an impressive five tries and troubled the Wests defence with every touch.
Following the 32-0 half time lead, the Eels added another two tries before the Magpies got on the board through winger Harrison Coskerie.
With the score out to 48-4, Magpies second-rower Eddie Kendall scored in the corner with just three minutes remaining.
Five-eighth Hose Leuea landed an impressive goal from the touchline to make it 48-10 in what looked as though it would be the final score.
However, the Eels went over again in the dying stages with Interchange player Cameron King helping his side crack the half-century.
Parramatta 54 (Griffiths 5, Fagaloa-Time, Tramontana, Doolan, Dargan, King tries; Fonua, Dargan goals) defeated Western Suburbs 10 (Coskerie, Kendall tries; Leuea goal)