Overnight lows will get down to 7C on Sunday night, 6C Monday night and between 9C and 10C in the middle of the week.
"Fairly mild really, people shouldn't expect frosts," Miller said.
"Sunday morning had a rainy start, but that's an improving friend for the remainder of the day."
Miller said the showers would ease later on Sunday and be fine by evening.
He said overnight on Saturday, the Whanganui airport weather station recorded 4.4mm of rain.
"Drips and drabs really, but it might have felt like more at the time."
Saturday got to a high of 16.3C, and an overnight low of 8.1C, so pretty average.