Officers were pelted with bricks, bottles and broken pavers as they attempted to break up two melees involving hundreds of people in the city's centre.
One officer sustained head wounds and another a shoulder injury as they arrested an alleged rioter near McIver train station in the early hours of Friday.
The violent incident followed a large-scale fracas involving about 150 people at the city's central train station on Wellington Street earlier in the night.
Video on social media shows dozens of riot police carrying shields charging the crowd across an intersection.
Assistant Commissioner Metropolitan Region Brad Royce said the incident started when a small group of people began fighting with staff at the station.
"Police went to the assistance of the transit officers and that quickly grew to around 150 fighting," he told reporters on Friday.
"A number of things (were) thrown at police ... mainly bottles and whatever could be picked up."
Asst Comm Royce said specialist officers helped disperse the crowd and seven people were arrested.
"It was pretty poor behaviour by members of the crowd, (many of whom) probably weren't involved in the first incident and saw it as opportunity to act out.
"The disappointing thing was, again police trying to do their duty and people stacking on who have nothing to do with it, just getting involved."
Police were also called to a brawl on Moore Street, near McIver train station, after one of the people involved was spotted with a weapon.
"We (also) had people picking pavers out of the ground and throwing them at officers," Asst Comm Royce said.
"Two of our officers were injured, luckily not seriously.
"One had a dislocated shoulder ... and one had minor facial injuries from a scuffle."
Asst Comm Royce said neither incident was related to the nearby Australia Day celebrations or protests earlier in the day.
"There is certainly nothing linked to protest activity or any established groups, or anything like that," he said.
"It was just really poor behaviour by a subset of the community who didn't go home with most of the crowd."
Police continue to investigate the incidents and review CCTV and body worn camera footage of the brawls.
No details about the arrests or charges has been provided, however, Asst Comm Royce said they are likely to be related to disorderly conduct and affray offences.
WA Police Union acting president Paul Gale condemned the attacks on police officers, which he suggested were politically motivated.
"If you have a problem with the politicians, take it out at the ballot box," he said.
"Don't take it out at police officers who are working unsocial hours and doing their best to protect property and keep the community safe."
In other states, police reported generally well-behaved Australia Day crowds with few incidents.
However, in Melbourne police were called to an incident about 10am in Coburg, where they formed a protective line between people attending an outdoor function and protesters.
A Victoria police spokesman said the event attendees moved the gathering to an indoor venue and the protesters moved on.
"Police will review body-worn camera footage to determine whether any offences occurred," she said.
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