Steel said tourist spending along with good returns from forestry, beef, kiwifruit, apples and wine are helping offset the extreme weakness in dairy, meaning New Zealand's external deficit has remained smaller than usual.
The PSI comes after its sister survey showed manufacturing continued to firm in May as production surged and employment expanded.
The composite index, which combines the two measures, decreased 0.8 to 56.9 on a GDP-weighted basis and edged up 0.1 to 57.4 on a free-weighted basis.
Employment was the only sub-index to improve in May, up 0.7 to 54.5, above its long-term average of 51.3. Combined with last week's manufacturing survey, Steel said overall labour demand looked buoyant and fitted the bank's forecasts for decent employment growth and a marginally lower unemployment rate in the second quarter.
Activity/sales fell 2.6 points to 63.2 and new orders/business was down 1.3 points to 60.1, though Steel said both had remained "particularly strong" as their index levels were above 60.
Supplier deliveries dropped 2.2 points to 52.1, while stocks/inventories dipped 0.1 of a point to 52.3.