With Keenan Hoedemaker through one-on-one with only goalkeeper Danny Knight to beat after a neat Onehunga passing movement, the Cashmere Technical stopper made the save of the game with his feet denying the midfielder.
Five minutes later Wellbourn and Barton combined to score twice in 60 seconds, emphatically killing off the contest.
Cashmere Technical coach John Brown hailed his team's effort.
"We knew it was going to be really difficult," Brown said.
"It's not often you come to Auckland for the semifinal of the Chatham Cup, and I felt the boys were really up for it.
"They worked their socks off and that's probably what got them over the line.
"You've got to try and get that first goal but there was a period at 2-0 when our 'keeper pulled off a great save and if they'd scored that, it would have been all on. But after that we started to get more space and I felt we deserved the result in the end."
Onehunga Sports co-coach Hiroshi Miyazawa said experience won out on the day as the first semifinal of the 91st edition of the ASB Chatham Cup was turned by the quick goals conceded in each half.
"We didn't have any luck, unfortunately," Miyazawa said.
"An early first-half goal and early second-half goal made it hard. Had we done anything wrong in conceding those two goals? One was a set piece and the other a penalty.
"Clearly they are an experienced team and we are a very young team and that's the difference between them and us. Physically as well, we're a fit and mobile team but it's hard to get through a team with those strong boys.
"Overall we've had a great season and I'm proud of the boys, they did everything they could do."
The second semifinal between five-time ASB Chatham Cup champions Central United and four-time winners Napier City Rovers kicks off at 1pm on Sunday at Kiwitea Street in Auckland.
