The BNZ-Business New Zealand measure of activity shows the region falling to 48.6 from 52.5 in March and 55.7 in February.
In April last year, it was 44.8, where a reading above 50 indicates expansion and under 50, contraction.
The seasonally-adjusted national index for April was 54.1, down 2.6 points from March but the sixth consecutive month in expansion.
Last month's reading was the highest April figure recorded since 2007.
National results for the various service sectors were a mixture of expansion and decline in April.
Property and business services (58.3) displayed strong growth with cultural, recreational and personal services at the same level of activity (58.3).
Wholesale trade (54.8) also showed expansion, while retail trade (46.3) and accommodation, cafes and restaurants (41.4) were both in decline.
BNZ markets economist Stephen Toplis said the service sector was a grouping of many and varied businesses often with extremely differing performance characteristics.
"From our perspective, the New Zealand economy is now almost 15 months into its recovery phase.
"This is an important point to note. The recession is now a very long time ago.
"That's not to say all is rosy as the expansion so far has only just filled in the hole created by the downturn."
Those doing the best included three of New Zealand's key export industries - dairy, logging and tourism, he said.
The gains achieved by those industries would be felt more widely in time but much of the current gain in revenue from those sources was price, not volume-related.
In the case of the core services sector, that meant many industry groups would see little immediate benefit.
Although the overall services index remained net optimistic, there were a few warning signs in the survey, Mr Toplis said.
"To start with, it represented a modest pull back from the 56.7 reported last month.
"But, perhaps more bothering, was the fact that activity, new orders and supplier deliveries fell at the same time that stocks and employment rose.
"At face value, this would tend to suggest that business hopes may have got a tad ahead of reality."
However, on balance, the survey's outcome was another piece of economic evidence that made Mr Toplis "guardedly optimistic" about the future.
