Although police praised the general good behaviour of Australia Day crowds at events across the peninsula, youths roaming around central Mornington, including Main Street, kept them busy.

Police handed out 22 notices for under-age possession of alcohol, fining them $65 each. Five under-age youths were brought back to the station and their parents were asked to collect them.

Nine adults were given $100 penalty notices for possession of unsealed alcohol. Another nine were brought back to Mornington police station lock-up and fined $489 each for drinking in a public place.

Sergeant Steve Duffee of Mornington police said it was a disappointing result for the 60 police officers deployed to Mornington to control the day's festivities. "We are displeased by the amount of alcohol seized and the number of under-age people drinking," he said. Most of the male and female youths caught with alcohol were not from the Mornington area, he said.

Sergeant Duffee said it was a timely reminder to parents - "you have to know where your kids are and what they are doing".

Police attendance is expected to be even bigger next year. "Next year Australia Day falls on a Saturday so there will be more people expected to join in the celebrations," Sergeant Duffee said.

"Every year we work closely with the council to form a plan of approach for Australia Day and we take a zero tolerance approach to alcohol and antisocial behaviour."

There was one notice given for offensive language and police are continuing to investigate an assault that resulted in a 16-year-old male victim losing a tooth.

In another incident, a 15-year-old male and a 16-year-old friend received minor injuries when they were walking home along the Nepean Highway after the fireworks display at Mornington at 9.40pm.

The boy was walking with three friends when they went past a group of five males who followed them and demanded they hand over their mobile phones.

One of the group threatened the boy with a bottle and another escaped with a phone. The group of five youths were apprehended by police 300 metres along the road.

They were all bailed after three were charged with robbery, affray and intentionally causing injury. The youths were also found to be in possession of jewellery and watches that are allegedly linked to a burglary in Mornington.