Another Tyrell penalty and try by Myers in the second half made Wanganui safe at 21-12 as the final quarter was a steady progression of scrums before Poverty Bay finally got some territory to cross after the 70th minute.
"They adapted well, they did bloody well," said coach Danny Tamehana.
"It was wet weather rugby - name of the game. Put the ball down there."
Tamehana said he has definitely see improvement from the side over the last two matches, although the side has not seen a firm dry track since their 26-19 win in preseason against Horowhenua Kapiti in Levin.
They will now travel back to Levin to meet the Wellington Samoan Under 18's this coming Saturday.