Stupidity led a New Zealand man to agree to inject his friend with heroin in a central Sydney hotel room, causing his death, a court has heard.

Daniel Gray, 39, at first resisted Gregory Wood's suggestion that they try heroin for the first time towards the end of the alcohol and drug-fuelled buck's night of a friend on January 4, 2009.

Gray pleaded guilty to Mr Wood's manslaughter in March this year.

At a sentencing hearing at Sydney's Downing Centre on Friday, Gray told the court he never should have given in when Mr Wood suggested buying heroin, something he'd been "adamant" he wanted to try.

Gray said "stupidity" led him to agree.

"I feel very responsible," he told the court, choking up.

"If I had just told him 'you're an idiot, let's just leave it' - if I hadn't given in to his whining, he'd still be here today."

The court heard that the two men were too intoxicated to inject themselves, so they injected each other at their Haymarket hotel room.

Mr Wood began to snore over about 40 minutes, and when his lips turned blue Gray realised something was wrong.

He tried to resuscitate him without success.

The court heard that Mr Wood had been on a three-day bender, and that night had snorted vodka and taken ecstasy and cocaine before injecting the heroin.

His cause of death was multiple drug toxicity.

Crown prosecutor Tony McCarthy said the men may have unknowingly bought a speedball, a mixture of heroin and cocaine.

Gray said he had been wracked with guilt over the death of his school friend.

"I don't sleep ... It's constant in my mind," he said, on the verge of tears.

"It's just always there, no matter what."

In her written statement to police, Mr Wood's wife Kirsten said she thought of Gray as a "bad arse" and "a bit of a rough diamond among the group".

Her statement said that Mr Wood "was easily persuaded when on a high with the boys".

"I know he was looking forward to (our) baby coming and may have thought it was the last chance for a blow-out."

Ms Wood reportedly asked that no charges be laid against Gray.

The hearing continues.