Fosita emerged from the bench to win the game in the 78th minute and lift Northland into a share of second spot in the second tier.
"Again the boys just stuck at it and came away with the win. It just shows the heart they've got," said Witcombe.
"That game was massive for us to stay in touch with the semis. If we hadn't won it would have been very hard work, and it was massive confidence wise.
"[Northland] seemed to show a bit of composure, that's what won that game. We didn't really play any rugby but it was a satisfying result at the end of the day.
"[Bay of Plenty] are a hard team to play down there, even in my career with Auckland we lost [often] down there." Much to Witcombe's delight was the way halfback Tane Takulua was instrumental in the win, scoring two tries from sniping runs around the ruck.
Witcombe agreed Takulua's running game was a strength that had been lacking of late.
Takulua had really impressed him. "He played out of his skin," Witcombe said. "We spoke to him about keeping his head up on the way to the ruck and assessing what is happening. His decision making is progressing too."
He said stand-in captain Dan Pryor again did the job and a Super Rugby contract was surely not far away.
Witcombe said Pryor had proved a suitable replacement for injured captain Cam Eyre.
"[Eyre's] leadership is a loss but DP stood up and did a good job, and the boys are listening to him."
The Taniwha's focus now shifts to Friday's home game against North Harbour, which could essentially guarantee them a semifinal spot if they win.
"The boys love playing at home, especially when everyone turns out to support us. It's our last home game, we have got to front up to the local crowd.
"We're going to be working hard this week and have a focus on ourselves rather than North Harbour," Witcombe said.
"Come along and help us, because it does really make a difference for the guys. I've said it over the past weeks, if we get our game right we can foot it with anyone."
The only injury concern to come from the game was a head knock to centre Brook Gilmore, who may be out if concussion is suspected.