Several others in their late teens were watching on, according to Mr Miller.
He said there was "no doubt" that alcohol was a factor in the lead up to the incident.
Mr Robinson was unconscious when police arrived. He was taken to Wairarapa Hospital and then airlifted to Wellington Hospital when medical staff realised how serious his injuries were.
He was operated on for bleeding on the brain.
Pioneer Club President Tere Torea is at Wellington Hospital with Mr Robinson's immediate family.
He said Mr Robinson is a "great player with great potential".
Tony Hargood, CEO of Wairarapa Bush Rugby Union, said the union's thoughts are with Mr Robinson's family.
He said Mr Robinson is an "elusive" runner, the top try scorer in club rugby this year and has played for the Wairarapa Bush Maori side.
The alleged offender Regan Brian Smith, 19, is also a member of the Pioneer Rugby Club.
Mr Miller said the reported clash hadn't been over anything to do with rugby.
Police have spoken to six witnesses who have been co-operative with the investigation.
Mr Miller said bystanders had tried to help Mr Robinson.
"It happened fairly quickly. A number of people close by tried to help."
There were no weapons involved in the altercation, according to Mr Miller.
He gave a warning to others about the influence of alcohol, which he said can turn an argument into a far more serious issue.
"Excessive alcohol combined with an argument can be extremely serious. It's another example of too much alcohol - serious injuries can occur.
"Fights these days can have far more serious consequences than first intended."
"This has turned out to be very serious for both the person injured and very serious for the person that's not. Those consequences will continue for many years for these young men."
Smith appeared in Masterton District Court yesterday charged with assaulting Sean Robinson on Sunday.
Represented by defence lawyer Jock Blathwayt, he was granted bail with conditions including a 24-hour curfew at a Carterton property and not to consume illicit drugs or alcohol.
He is also not to contact Robinson or any members of his family.
Smith is due back in court on June 23 to plead to the charge he faces.
Police are looking for any further witnesses of the alleged assault to come forward.