Unfortunately for the Sky Blues, West Coast had another 40 minutes of gas in their tanks, winning the second half 31-5.
The East Coast points came from tries to winger Epeli Lotawa (2), fullback Morrison Siliko and flanker Trent Proffit, along with three conversions to first five-eighth Ngarangi Haerewa.
The four tries gave East Coast a bonus point — their second of the championship.
Despite the victory, West Coast missed out on a place in the Lochore Cup (for fifth-to-eighth-placed teams) after having six points deducted for an eligibility breach in previous games. One of those games was a 31-20 win against Poverty Bay.
Loss of the points meant West Coast finished on 16 — the same as the Bay and Buller — but West Coast finished above them on points for and against.
Top qualifiers and defending Meads Cup champions Wanganui will host Thames Valley in one semifinal on Saturday, while South Canterbury are at home to King Country in the other semi.
In the Lochore Cup play-offs, Horowhenua Kapiti (fifth) will host Mid Canterbury (eighth).
North Otago (seventh) beat Wairarapa Bush (sixth) 24-0 in Masterton but failed to score a bonus-point try and will return to Masterton for the second semi.
Round 8 results: Wairarapa Bush 0 North Otago 24, Wanganui 57 Horowhenua Kapiti 27, South Canterbury 33 Thames Valley 24, Poverty Bay 27 Buller 41, Mid Canterbury 29 King Country 31.