Temperatures during the month ranged from highs of 15C to 22C, and lows of 7C to 13C.
The highest temperature, 23C, was recorded on April 12. The lowest was 6C, on April 4.
Winds were mainly west and northwest, with an 85km/h gust recorded on April 13 the strongest.
Whanganui wasn't alone in its dry conditions. Large parts of the North Island are in drought, Niwa meteorologist Ben Noll said, especially the north and east. For some regions it is the worst on record, rivalling 2013.
The dry weather was caused by persistent high pressure systems in the north Tasman Sea suppressing rain-bearing systems arriving from the south.
Niwa has predicted northern and eastern regions of New Zealand will get drier under climate change, with the drying effect increased by the higher temperatures of global warming.