Captain Mitch Marsh won the toss and elected to bowl first, which looked to have been an inspired decision when Andrew Tye removed Jack Edwards for 12 runs.
However, WA didn't have an answer for the Blues' other opener Dan Hughes who racked up 152 runs to help his side to 6-348.
Tye claimed three wickets from his ten overs, Nathan Coulter-Nile returned figures of 2-59 while Marsh was economical leaking just 38 runs from eight overs.
West Aussie opener Josh Philippe got his sides chase off to a flying start, cracking 64 runs from 50 deliveries which included three sixes and seven fours.
A middle order collapse for WA saw them in trouble at 5-103, before Hilton Cartwright tried his best to launch a rescue mission with Ashton Agar.
Cartwright looked solid for his 74 of 68 balls, while Agar supported him well before he was out caught for 30.
NSW eventually dismissed WA for 271, with the loss meaning Western Australia slip to second spot on the Marsh One-Day Cup table.