From Monday, about 50 riot squad officers will carry M4 semi-automatics after completing an intensive 10-day training course.
The move was first flagged in June after police were granted greater shoot-to-kill powers following the recommendations of the Lindt Cafe siege inquest.
Police Commissioner Mick Fuller on Monday admitted the guns would cause some concern in the community.
"We have to get used to it," he told reporters in Sydney.
"There's no point in us pretending the environment hasn't changed - the organised crime environment, the terrorism environment."
By June 2018, the entire riot squad - about 100 officers - will carry the rifles.
The move comes after police in Victoria thwarted an alleged plot to use an automatic rifle to shoot many people in Federation Square on New Year's Eve.
International Security Expert James Der Derian told SBS News the switch from a hand-held Glock pistol could result in unintended damage.
"You increase the range and accuracy, but you also increase the velocity," he said.
"So the danger is that theoretically (there’ll) be bullets passing through rather than stopping somebody. So you may be talking collateral damage on the streets of Melbourne or Sydney."
NSW Police Commissioner Mick Fuller and Police Minister Troy Grant (right) arrive to a media conference (AAP) Source: AAP
The University of Sydney academic said the upgrade introduces an element of "militarisation" on to Sydney streets.
"We’ve seen this previously and you always have to ask does the threat warrant this response and that’s based on intelligence," he said.
"If there’s new intelligence that I’ve not heard or seen, and then yes it’s warranted. But if you’re really talking about the lone-wolf war, as has been the case in Australia more often (caused by) mental illness as a trigger (for) these events, I’m not sure it’s going to have a deterrent effect."
NSW Police is yet to answer questions about how much the guns and training cost.
The state's 16,000 officers have also received active armed offender training and the weapons will, in general, be reserved for those high-risk situations.
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Deputy Commissioner Dave Hudson warned they may be deployed to large gatherings, such as New Years Eve events, with the commissioner's approval.
"That will be an assessment made on a case-by-case basis," he said, citing the significant deterrent effect of the firepower.
Mr Hudson said most overseas incidents involved distances of over 25 metres and the current Glock firearms were not effective in those scenarios.
The M4 weapons are already used by the tactical operations unit and were the guns involved in the response to the 2014 Lindt Cafe siege.
-With AAP
