Waikato will stay in the provincial rugby Premiership, while Bay of Plenty have been condemned to relegation.
Bay of Plenty scored three tries to one in Saturday afternoon's clash in Tauranga - but Waikato still came from behind at the death, to win 28-25.
Bay of Plenty got in front for the first time before half-time, thanks to two tries to flanker Johan Bardoul, the Steamers leading 15-12.
Then they were up 22-12 just after the break through a try to right wing Maritino Nemani.
But Trent Renata kept Waikato in touch, kicking seven penalties to take the lead back before centre and captain Jackson Willison scored the match-winning try in the 77th minute.