Melbourne will test a terrorism alarm system this month after police thwarted two alleged plots targeting Christmas-New Year crowds and a lone driver killed six pedestrians in the last year in Australia's second-largest city.

Loudspeakers are being installed at more than 90 sites across downtown Melbourne as part of the alarm system that will be tested on Dec. 28, Victoria state Acting Police Chief Commissioner Shane Patton said.

The speakers will sound a siren and police instructions to the public in the event of a terrorist attack.

Patton told reporters in Melbourne, the state capital, that even though Victoria "is essentially a very safe place," action must be taken to "make sure that we are resistant to terrorism."

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Police last month charged a 20-year-old Australian-born citizen with Somali parents with preparing to commit a terrorist attack in Melbourne. Police allege he planned to use an automatic rifle to shoot ad many people as possible in downtown Federation Square on New Year's Eve.

Four men are waiting to stand trial on terrorism charges after they were arrested in December last year over a similar alleged plot to attack Federal Square and other public places in Melbourne over the Christmas-New Year period. The four will stand trial next year on terrorism charges.

A 27-year-old man has been charged with six counts of murder and 28 of attempted murder after allegedly driving a stolen car along a Melbourne pedestrian mall on Jan. 20. His lawyers have told the court he is unlikely to be mentally fit to enter pleas.