"He's been taken to hospital for observation but he's going to be okay."
Police were called about 8.30pm but had been unable to find the offender.
The man was taken to hospital with minor injuries, a police spokeswoman said.
Police are continuing to make inquiries to identify the attacker.
Three of the arrests were for pitch invasions - one person was arrested before they could get on to the field and two others went onto the ground after the final whistle.
"Under the Major Events Management Act, going on to the playing surface at Rugby League World Cup matches is an offence punishable by a fine of up to $5000 or imprisonment for up to three months," said Inspector Karen Henrikson, RLWC operations commander for Waikato Police.
"Police won't tolerate people going on to the pitch and we will charge anyone who is silly enough to do so."
A fourth person, who was drunk, was arrested and taken away to sober up and another 20 were evicted from the venue by police and/or security for intoxication or bringing in prohibited items.
Despite the incidents, police and security guards were very impressed by the crowd.
"Considering all the hoopla that we thought was going to happen, this is probably one of the best crowds we've had to deal with at Waikato Stadium," said head of Waikato Security Chris Mangan.
"It's been great to see something so different at the stadium, and we've been doing work here for nearly 20 years."
Henrikson said the crowd had been boisterous and vocal.
"It was a really good crowd, with plenty of singing and plenty of colour."
Samoan and Tongan league players joined together in a prayer as a sign of unity shortly before kick off.
The match between the two South Pacific neighbours had been the most anticipated of the tournament.
Tonga won the game 32-18.
Meanwhile, in South Auckland, spirits are high as Pacific Island rugby league supporters take to the streets to party after the match.
At least a few hundred people are gathered on Atkinson Ave in Otahuhu, a Herald photographer at the scene said.
And while most are Tongans celebrating the island nation's defeat of Samoa at Waikato Stadium tonight, Samoan fans are also there.
Some revellers played music while others in vehicles tooted their horns.
The photographer said the road was heavily congested and police were in attendance, although nothing "untoward" had happened.
The celebrations follow a week of violence as fans from the rival teams clashed in New Zealand's Pasifika heartland of South Auckland.
Tensions flared on Thursday night when up to 200 people were involved in a brawl between opposing fans in Otara.
A young fan was seen burning a Samoan flag and others were understood to have been armed with weapons including a machete, pole and sticks.
The burning triggered a crowd of stick-and-pole welding fans to fight each other in a carpark at Otara Mall.
Six people were arrested for disorderly behaviour and possession of offensive weapons, including a machete, police said.
