An assault of West Australian Premier Colin Barnett's 26-year-old son Sam Barnett in a Northbridge nightclub is being investigated by police.

Sam Barnett posted on social media that he was punched in the face "three or four times" in an unprovoked attack on Saturday night.

He described the assault on Facebook the following morning.

"Assaulted in Northbridge last night after the polo (I've probably been to Northbridge 5 times in my life). Was punched in the face 3 or 4 times. Was completely unprovoked and random," he posted.

Mr Barnett said he was leaving the bathroom at the venue when he was hit in the head.

"Walking out from the bathroom someone hit me square in the face, which knocked me back, fortunately there was a couch there," he said.

"The person who did that proceeded to lean over me and kept punching me in the face.

"I managed to slide out and leave the club as quickly as I could."

Mr Barnett said it was an unprovoked attack.

Loading

"The natural assumption is that I was doing something wrong, or was too intoxicated but it was truly an unprovoked attack," he said.

"I'd never seen him before in my life, never spoken to him, never seen him before in my life, didn't do anything silly like speak to his girlfriend, all I did was literally use the bathroom."

Mr Barnett said he was disappointed in the way the club dealt with the incident and that he felt under pressure to tell the police to leave.

"The staff I found extremely intimidating... I was actually thinking there was going to be a second assault, there was about 15 people, friends of the alleged perpetrator and also the staff, at that point," he said.

"I stood behind the police because people kept coming up to me saying 'just forget about it, we'll buy you a drink'."

Mr Barnett said he was lucky not to sustain more serious injuries.

"I was very lucky, I think I'll pull up in the morning with a few bruises and I have a very sore head, but the inside of my lip was gushing with blood. There is no way I was going to accept my 'free drink' and leave," he said.

Mr Barnett said he had gone to the police station and had filed a report and the police had identified the alleged offender.

He said he would be engaging a lawyer on Monday and he hoped the matter would be taken further.

A WA Police spokeswoman confirmed officers were investigating an incident where a 26-year-old Claremont man was assaulted at a nightclub on Roe Street in Northbridge.

The Premier's office has been contacted for comment.