Acting Police Chief Commissioner Shane Patton said police chose not to activate the alarms last Thursday when a driver allegedly drove into a crowded intersection, injuring 18, as they were able to quickly contain the incident.

"It was considered but because we very quickly could control the scene, there was no necessity to do so. We were able to use other avenues, other media, to put out what the situation was," said Mr Patton.

The sirens, which were tested for the first time on Thursday, are intended to alert and inform the public of what to do during a violent incident in the CBD. There are currently 65 speakers across the city.

They may be used if there are terror attacks, sieges, riots or vehicle attacks, and will include instructions such as, stay in place, move to a safe location, or stay away from windows.

However they may not always be necessary or helpful.

"If we have a vehicle-borne attack these can happen very quickly, they can be a very short period of time and it might not be appropriate to use this system. But if we have something like an active armed offender incited or an ongoing incident we would then utilise it."

Mr Patton said last January's alleged vehicle attack in the CBD's Bourke Street, which killed six people, may have been an appropriate time.

"If we had the system then it could possibly have been used in the Bourke Street incident... given the time span that it occurred over."

Police Minister Lisa Neville said this system would be one part of a large suite of security initiatives.

"We will be adding an additional 30 sites by the end of next year, in addition we are putting in place the permanent bollard systems," said Ms Neville.

"All up, the package at the moment is $10 million... to enhance the security measures in the CBD."

Although today's test of the alarms barely overpowered the sounds of passing trams, Mr Patton confirmed that during an emergency they would be louder.

"They will ramp up depending on the incident. They are able to get very loud."

Saeed Noori, 32, has been charged with 18 counts of attempted murder and one count of reckless conduct endangering life, over the Flinders Street incident.

Tensions raised by that incident were increased that day with reports of gas bottle explosions at a Frankston shopping centre.

A week later, on Thursday, police said they had arrested a 57-year-old man who was helping them with their inquiries.

"The incident occurred in the loading dock at a shopping centre on Beach Road about 7.10pm on December 21," police said.

There were no injuries from that incident.