Kain Parsons was critically injured in a match against former Canterbury and Tasman Makos rugby halfback Steve Alfeld during the Fight for Christchurch event at Horncastle Arena last night.
Parsons was not wearing protective headgear, a witness said.
A spectator who was sitting ringside told the Herald the incident was "awful".
"He went down with a short clean blow, didn't appear too aggressive but when he didn't jump up within a few seconds it was clear it was more serious."
Emergency staff went straight in and were "amazing", she said.
Not many of the fighters wore protective head gear.
"I was surprised more contenders weren't wearing head gear. I know it's only amateur but surely that's more reason to make sure the fighters are fully protected."
Emergency services were called to the arena just after 11pm, and Parsons was taken to hospital in a critical condition.
A Canterbury District Health Board spokeswoman said the family was requesting privacy and would not be providing any comments.
Event promoter Callam Mitchell said they were "devastated" following the incident.
"All of the team behind the event are absolutely devastated by what has happened and our thoughts are solely with Kain and his family at this time.
"Our hearts also go out to his coach, contender, and work colleagues."
According to his fighter biography, Parsons was a former builder who now worked in project management at Versatile Garages in Christchurch.
He was a "keen spectator" at the 2017 charity boxing event, and this year decided to give it a go and raise funds for the Conducive Education Canterbury, an early intervention programme for preschool children living with developmental delays and disorders.
His Givealittle page set up for the fight had raised nearly $3000 as of this morning, with a target of $5000.
Fight for Christchurch involved 18 amateur boxers fighting to raise money for two charities.
The fighters are described on the event website as "ordinary people, who have stepped up to do something extraordinary".
"One step at a time, one punch at a time, one round at a time, our contenders will shed sweat, tears and even a little blood to prepare themselves for their bouts."
Each boxer was championing either the Cure Kids or Conducive Education Canterbury charities.
The charity event has raised hundreds of thousands of dollars for Christchurch-based charities over the past eight years.
Conducive Education Canterbury said in a statement they were "heartbroken" by the news of Parsons' injury.
"We are heartbroken at this turn of events and wish to extend our utmost sympathy to Kain and his family and are praying for his recovery.
"Our thoughts also go to his opponent Steve Alfeld – both of these men have big hearts and have supported us tirelessly.
"This is a tragedy for all involved and our thoughts are with them and their families."
