The fatal incidents within 40 minutes of each other came hours after officers in Newcastle fired on a man wanted by police on Tuesday morning.
At an inner-Sydney unit complex, a knife-wielding offender died after a police office opened fire.
The man had entered a unit complex in the upmarket suburb of Potts Point and assaulted two women in separate units about 10.50am.
Arriving officers used a taser to control him but it was ineffective, police said.
The alleged attacker then confronted police before an officer fired one shot, hitting the man, who died at the scene, Assistant Commissioner Peter McKenna said.
The women, aged 48 and 56, suffered facial injuries and were taken to hospital.
Mr McKenna would not speculate on why the stun gun was ineffective.
"Tasers have been an effective part of our arsenal," he told reporters on Tuesday.
"They are a less-lethal option but like anything, they're not always successful.
"(Police) don't come to work to be confronted by this type of thing or take a life - that's the last thing we want to do."
He emphasised officers had been responding to the women in distress when the situation escalated quickly.
A crime scene was established with officers using sheets to cover the dead man, who is believed to be aged in his late 30s to early 40s.
The street was cordoned off with a helicopter flying above the scene and up to 20 riot cars, police vehicles and ambulances descending on the block.
Nuria Cobos, who has been living down the quiet street for about a year, described the scene as confronting in a safe neighbourhood.
"I feel lucky. I went out for a walk and came back to see all of this," the 40-year-old woman told AAP.
With several detectives entering the street in front of the HMAS Kuttabul navy base, she was waiting to be let back in along with confused food delivery drivers.
Forty minutes later, police responding to a welfare check request shot dead a man in his Tingalpa home in Brisbane's east.
Officers had attempted to negotiate with the 21-year-old man but he made threats with a knife towards officers and was shot, police said.
He died at the scene with one officer suffering a minor injury
"These incidents are often dynamic ... and split-second decisions need to be made," Acting Chief Superintendent Heath McQueen told reporters.
"I am confident that the use of force is appropriate."
The matter is also being investigated with oversight from Queensland's Crime and Corruption Commission.
In a third incident, a man was shot as police from a high-risk domestic violence offender squad tried to arrest him in inner-city Newcastle.
The officers forced entry into a unit at 8.30am and found the man - wanted on five warrants - armed with an edged weapon, police said.
Two stun guns were deployed but both were ineffective.
The man, 36, allegedly continued threatening officers before a police officer shot him in the shoulder, police said.
He survived and was taken to hospital in a stable condition.
Both NSW incidents are being investigated by out-of-area police before being reviewed by the internal police investigative unit.
Each investigation was overseen by the state police watchdog.
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