A 28-year-old man is on life support and his high school sweetheart facing a future without him after a violent attack at a West Auckland bar.

Surgeons have removed half of his skull to relieve pressure on his brain but his head injury is so severe it is unlikely he will survive after what is said to be an unprovoked attack at The Mates Bar on Friday.

The victim is in hospital surrounded by family. His mother-in-law spoke to the Herald this morning.

"It's heartbreaking," she said.

"We've been told he most likely won't survive this.

"My daughter … she's broken, this is her love, they have been together since high school - they thought they were going to have babies, they thought they were going to get married.

"He was supposed to come home from work on Friday and he never did."

The victim, who the Herald has agreed not to name, works as a roofer and was on a job in the Kumeu area last week.

"The finished early and went for a beer with their boss," the mother-in-law said.

"Their boss had just left and (the victim) called (a mate) to come and pick them up. It was about 4.30pm.

"(The friend) rang back but there was no answer, the phone just kept ringing."

The alleged incident was happening as the friend tried to call.

It is understood the victim's friend was attacked first and he was set upon as he tried to help.

"These are good young men, hard working boys - they are not vicious, they are not violent, they have their whole futures ahead of them, they had a good day at work.

"(The other victim) is not an aggressive person at all… I spoke to him yesterday and he is devastated, you can see it in his face.

"This boy has to live with this for the rest of his life, he keeps thinking 'what if I had done this differently'."

The victim underwent emergency surgery and his family will meet with doctors early this afternoon for an update.

"But they have done everything they can for him," said his mother-in-law.

She said in her opinion the attack on the victim and his mate was "senseless and disgusting".

"It was random, and it shouldn't have happened," she said.

"This is all so surreal. These people cannot get away with it."

Detective Senior Sergeant Kim Libby confirmed police were investigating an alleged incident at the bar.

"One man received critical injuries in the assault and a second man received moderate injuries," he said.

"Police have arrested a 38-year-old man who is due to appear in the Waitakere District Court tomorrow charged with wounding with intent to cause grievous bodily harm.

"Further charges and arrests are likely.

"A scene examination has been completed and enquiries are ongoing," he said.

"Police are asking anyone who witnessed the incident to come forward and speak with them."

Anyone with information can call 105 and quote event number P044313222.

A woman who identified herself as an owner of the bar refused to comment.

"At the moment it's all fabricated and quite twisted, so no comments will be made at the moment," she said.

"I mean the people who are involved maybe making comments, but we'll get back to you as soon as the actual truth is there.

"It's in your best interests that you report absolutely nothing because what's come through is not factual."

The woman said she would respond to the Herald "within the week".