An official was taken to hospital after he was injured avoiding plastic bottles thrown by fans at an NRL game in Sydney.
Canterbury Bulldogs supporters targeted officials as they left the pitch after Friday's 18-17 defeat by South Sydney Rabbitohs, following a controversial late penalty against James Graham.
New South Wales police said another official injured a shoulder in falling.
They have identified two people for "allegedly throwing bottles".
Two other people were arrested on suspicion of assault, police said, adding: "Police will be taking legal action against them. Inquiries to identify others involved are continuing."
The match was played in front of more than 40,000 fans at Sydney's ANZ Stadium.
Bulldogs coach Des Hasler said: "This is not what this club is about.
"I've been to see the official who was struck by a projectile. He's okay."
NRL head of football Todd Greenberg said: "What we saw at the end of today's game was unacceptable."
The Bulldogs led 17-16 when referee Gerard Sutton awarded a penalty against England international Graham for a late tackle on Adam Reynolds.
Graham was placed on report and Bryson Goodwin kicked the penalty which gave the Rabbitohs victory in game between last year's Grand Finalists.
Reynolds was later diagnosed with torn knee ligaments and could be sidelined for up to five months.
Graham said: "I was thinking the decision was incorrect. It's probably not good behaviour from us in terms of crowding the referee."