Warren Shane Collins has pleaded guilty to wounding with intent to cause grievous bodily harm and faced a sentence hearing in Sydney's District Court on Thursday.

Crown prosecutor Henry Hudson told Judge Chris Craigie that Collins had been taking methylamphetamine in the weeks leading up to the assault at a Whalan property in August 2018.

Collins, who gave evidence via video link, thought his brother had been spiking his food with the drug.

He agreed with Mr Hudson's suggestion that he believed his brother was trying to sabotage him by putting the substance into his food - and also hack his phone.

He retrieved a small knife from his cupboard and walked into the living room where his brother was sleeping before stabbing him nine times. His sibling then fell to the ground.

Collins agreed his brother asked for help while lying on the ground.

"You gave him the phone more than two hours after the attack started ... you left him lying on the ground injured and helpless for at least two hours," Mr Hudson suggested.

The 51-year-old Collins agreed, saying he was "very, very sorry for what I have done to him".

Collins' lawyer said her client tried to kill himself on the day of the attack by stabbing himself and drinking hydrochloric acid.

A mental health assessment after the attack found Collins has schizophrenia.

He has since undergone a number of operations and will be fed through a nasal tube for the rest of his life.

His impairment was a reason he was unlikely to re-offend, his lawyer said.

The case has been adjourned to July 31 to enable a psychiatric report to be obtained.

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Australian Associated Press