Kylie Ellina Truswell-Mobbs, 50, was arrested more than a year after the death of David Ronald Mobbs, 56, at Alexandra Hills, in Brisbane's southeast, in December 2023.
Police said Mr Mobbs was in the "last stages of his life" with motor neurone disease and had been in and out of hospital before his death.
But Truswell-Mobbs "took unlawful actions in administering lethal levels of prescription medication to accelerate" Mr Mobbs' death.
Motor neurone disease, also known as MND, is a terminal nervous system disorder that rapidly damages a person's ability to walk, talk and eat over time.
Police said on Thursday there had been preliminary discussions between Mr Mobbs' family around assisted dying, but that option had not been formalised.
"There is a process you need to go through with assisted dying and in this situation none of those processes were followed," Detective Acting Inspector Rod Watts told reporters.
"Our evidence and investigations lead us to believe it is the offence of murder and not assisted suicide."
Police will allege toxicology and forensic tests show Mr Mobbs had lethal levels of his prescribed pain medication in his system.
"The acceleration of death, I can say, was limited to hours after he was administered the prescription medication, within hours he was deceased," Det Insp Watts said.
Police said Truswell-Mobbs lived with Mr Mobbs and had been in a relationship for years but was also one of his carers along with family members and a care agency.
Det Insp Watts said Truswell-Mobbs had spoken to police and investigators would claim there was a motive for the alleged murder.
But he would not disclose details of those elements of the case.
Det Insp Watts said police would not claim there was any domestic violence in the alleged murder.
It had taken a long time to charge Truswell-Mobbs with murder due to the case's complexity and forensic testing required, he said.
"It's not your straight-up homicide that you normally investigate. These things take time," Det Insp Watts said.
Officers responded to the "suspicious death" just before 10pm on December 7, 2023.
Both Mr Mobbs' family and care agency had reported his death to police.
"It's quite a traumatic experience for the family. It's a very emotional thing. It's a very sad story unfortunately. Police will put support in place," Det Insp Watts said.
Truswell-Mobbs was arrested on Wednesday after officers attended an Alexandra Hills address, police said.
Her case was briefly mentioned in Brisbane Magistrates Court on Thursday but she was not required to appear.
Her solicitor Andrew Hanlon did not apply for bail and sought an adjournment which police prosecutor Sergeant Duncan Erskine did not oppose.
Magistrate Rosemary Gilbert adjourned the matter to April 28 and remanded Truswell-Mobbs in custody.
Defendants charged with murder in Queensland must apply for bail in the Supreme Court.
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