It was a freak incident, he said.
"I don't think it's through any act of stupidity or foolishness, I think it's just bad luck."
While many of the people who were in the five or six trucks involved in yesterday's trip were Canterbury club members, it was a privately organised trip and not a club event, Mr Sergeant said.
Mr Brons was not a member of the club, but he was "quite a good friend" of those involved.
"Where it really hits home is the thought that pretty much everyone there does it every weekend and it's something that we're really passionate about, and when something like that happens it really hits home."
The people who took prt in yesterday's trip were "pretty cut up" about what happened, he said.
The club had undertaken trips on that route, known as The Chasm, before.
"It's a track we've done several times and we haven't had any issues with it," Mr Sergeant said.
The other people in the group had "years of experience", he said.
"None of them are new to it, and they're all pretty sensible fellas. They know what they're doing and they're [driving] really well set up trucks."
Tony Burgess, past president of the New Zealand Four Wheel Drive Association, said while it would be the passenger's responsibility to get out of the car in such a situation, it was an unusual move.
"Whenever we've had vehicles in a river, when there's any possibility of getting stuck, we would usually tie the rope on beforehand and let the vehicle tow the rope across so it can be winched out without anybody having to go into the river."
He described river crossings as "inherently dangerous", and said the main risk was the vehicle floating and then rolling over.
Someone getting swept away was "extremely unusual".
"I've been involved for about 20 years at the national level and this is the first time I've ever heard of [anything like this]," he said, adding they had "never had anything close to a fatal accident in 25 years".
"There'll be a huge number of people looking at themselves to see what went wrong and to see what can be done to make sure it doesn't happen again of course."
Mr Burgess had been on that route himself, and said it was "not particularly challenging at all".
"I've only been on it in summer admittedly, but there are other crossings around it that various clubs do that are known to be tricky and treacherous, but that's certainly not one of them."
Combined 4WD Clubs chairman Nigel Bannan said the four-wheel drive community was "quite concerned".
"I haven't heard, in the last 20-odd years I've been four-wheel driving, of someone being swept down a river and lost," he said.
"The four-wheel drive community's extremely shocked - hopefully they can find him."
A search for Mr Brons was called off at 5pm today with no sign of the missing man, police said.
A helicopter with a white water expert on board had scoured the remote country inland of Ikamatua this afternoon. Witnesses were also interviewed about the incident today.
A review on continuing the search would be conducted tomorrow, police said.
