But Napier scored immediately after a lack of concentration at the kick-off from Tauranga let the visitors in.
Barnstorming hooker Mafi Fifita restored the margin after he crashed over. Forbes added the extras for a 17-5 lead.
But the last 15 minutes of the half was all Napier, who used their strong set-piece to good effect into the wind. Some smart work from their outside backs and a gifted intercept saw Napier cross the line for three tries and a 26-17 lead at halftime.
Turning into a stiffening breeze it looked a difficult assignment for Tauranga, who were resplendent in bright pink socks to celebrate the role played by their mothers.
An early charge down the touchline by winger Sione Faiva nearly produced a try for Tauranga, who were kept in their own half for long periods by a dominant Napier team.
Staunch defence was called for and Tauranga managed to keep Napier out. On a rare attacking raid Forbes cleverly skipped through the Napier defence to score under the posts.
A perfectly executed lineout drive resulted in a second try for hooker Fifita and a lead of31-26. Forbes added a penalty on fulltime to end an absorbing match.
Tauranga coach Dan Goodwin was extremely pleased to keep Napier scoreless in the second half.
"It was a really important win for us against a very good Napier Boys' side as far as Super 8 goes as it puts us up the table," he said.
"I am very proud of the boys and mainly because they identified what needed to be fixed and they did the job. We let some pretty soft tries in before halftime but the boys showed some real steel to get us back in the game and also to hold out at the end down to 14 (men).
"There is heaps to work on. The tackle accuracy in the first half was a real concern but we got the job done in the end and the five points. You have to enjoy these ones as there are no easy games."
Tauranga Boys' have two home games coming up. They face Palmerston North Boys' this Saturday, followed by a match against Hamilton Boys' on July 8 which will be shown live on SKY TV.
