From as early as 2pm, dozens of fights were reported throughout the Illawarra with the largest occurring in Shellharbour, Kiama, Wollongong and Thirroul.

Most of the brawls featured between 30 and 40 drunken teenagers, many of them swathed in the Australian flag.

Wollongong police had prepared weeks earlier for the record 60,000-strong Australia Day crowd.

A 12-hour street operation took place, targeting three distinct precincts and using 44 extra officers from Lake Illawarra command and highway patrol units.

But by late afternoon, police were calling for back-up from Lake Illawarra and the operational support group - or riot squad - in Sydney.

From as early as 7.30am, teenagers began gathering to drink alcohol at the Wollongong foreshore, despite it being an alcohol-free zone and patrolled by 22 security guards.

Resident Grant King was leaving a Burelli St gym about 3.30pm when he saw a group of teenagers, decked out in Australian flags, turn into an ugly, angry mob. Mr King said they were "blind" drunk.

"They were carrying drinks, smashing them in the middle of the road and stopping traffic," he said.

"They were swearing and threatening any onlookers, including an elderly couple whom they spat at and asked, 'What the f... are you looking at?"'

In Kiama, a fight between 40 youths also broke out. It ended when a teenager was treated by ambulance officers after being king hit.

Police didn't escape injury either: three Wollongong officers were assaulted in various brawls throughout the day and five Shoalhaven officers were treated for minor injuries after a brawl at Huskisson.

In Thirroul, a transit officer was assaulted when he tried to stop a brawl between about 40 youths.

Inspector Stephen Hegarty said the 9pm fireworks at Wollongong Harbour were brought forward 10 minutes so the crowd could be moved on early.