A teenager has had his leg amputated after being hit by a car during a wild brawl in Melbourne, as police reject claims they were underprepared for the mass violence.

An 18-year-old man suspected of hitting the victim, also 18, has been arrested and released pending further injuries into melee of about 200 people outside Collingwood's Gasometer Hotel early on Sunday.

Prime Minister Scott Morrison questioned the effectiveness of Victoria Police, highlighting the involvement of African-Australian youths and saying "we don't have these problems in NSW".

Police Association secretary Wayne Gatt says officers at the scene were under-resourced, a claim denied by Acting Deputy Commissioner Bob Hill.

"Our members at a local level say they requested additional resources and were told that the response would be business as usual," Mr Gatt told reporters on Monday.

No one was arrested at the scene after fighting broke out at the 66 Records launch and spilled onto the road, but Mr Hill rejected suggestions police were not adequately equipped to deal with the crowd.

"We had the capacity (to make arrests) but, again, I'm not going to judge the tactical decisions made by Victoria Police," he said.

"To suggest that Victoria Police was not prepared for this event, to suggest that our rostering wasn't adequate, to suggest that we've got a no arrest policy is absolute nonsense."

Seven people were hospitalised, including the 18-year-old Fitzroy man who "has lost his leg as a result of these despicable actions on behalf of the driver".

The injured teen remains in the Alfred Hospital in a critical but stable condition.

He was a witness in the alleged murder of 19-year-old Laa Chol during an out-of-control CBD party in July.

Police on Sunday claimed they were caught off-guard by the weekend record launch, attended mostly by Pacific Islander and African-Australian youths.

"Some of the local police have indicated they were concerned about that, that's something I think police will need to absolutely have a look at," Police Minister Lisa Neville told 3AW radio on Monday.

Earlier in the day, Mr Morrison criticised police efforts in the city.

"We have (Australia's) highest proportion of Sudanese population in NSW," he told the radio station.

"We don't have these problems in NSW, they're not happening in other cities. Why is it happening in Melbourne?"

Before the weekend's event, organiser J-Nelly on Facebook urged revellers to be on their best behaviour, saying "violence will not be tolerated".

J-Nelly was contacted for comment, while the Gasometer Hotel released a statement saying they were cooperating with police.