Championship bouts take place on Friday and Saturday.
Joining Oldehaver are Isabella Parkes, 13, Tekahui Spittal-Rahina, 15, Chille Palmer, 16, and Pheenyx Apiata-Cook, 14.
Tofa said the ongoing Covid-19 pandemic had impacted overall numbers, but there would still be 70 bouts over the entire competition.
"Things have been going well. There was a real buzz yesterday [Wednesday] when things finally got under way.
"The level of competition is right up there. It's New Zealand title on the line so nobody wants to stand back.
"There are two people standing in the middle of the ring having a go at each other."
If Whanganui is to take home titles, it will be a quick turnaround before they have to defend them.
The River City will also host the 2022 championships in October.
Tofa said he expected the next edition to have a lot more entrants.
"Everything is already booked and ready to go. People are very happy to be here this time around and they want to come back.
"This competition is good practice for us, and a trial run for the next one. We're excited."
The New Zealand Amateur Boxing Championships run until April 23 at Jubilee Stadium.