"A straw case, a case that was built with absolutely no foundation," Leah Rowan said in the defence final address in the NSW Supreme Court on Monday,

Charlie Younes, 45, has pleaded not guilty to murdering 18-year-old Steven Copo-Horton, also known as Steven Copo. Steven died about a week after his mother, Rebecca Horton, broke off her relationship with Younes.

Steven could not walk or talk and moved around by crawling and shuffling on his bottom.

Younes said he discovered Steven in the bath of the mother's home and raised the alarm to those sleeping in the house about 2am on October 28, 2013.

Ms Rowan on Monday said when the Crown case was carefully scrutinised it did not make sense.

"You might think it unlikely, highly unlikely, that the deceased did all the acts necessary to accidentally drown," she told the jury.

But "highly unlikely" events happened every day.

She gave examples including that when the trial started six weeks ago, everyone would have thought it highly unlikely that "we would be scrounging for toilet paper a few weeks later".

She noted the Crown contended it was highly unlikely that a string of events - including Steven getting out of his bedroom, moving past his wheelchair, getting in the bath, turning on the water and accidentally drowning - occurred without him making any noise.

"If it is unlikely the deceased would do it, it is absolutely ridiculous the accused did," she said.

She referred to 14 events Younes would have had to have done without making any noise - including breaking into the house, getting Steven out of his room, running the bath and putting him in the tub.

He also would have had to, apparently by only using his fingertips, "hold a completely unresisting Steven under the water until he drowned while making absolutely no noise", she said.

While the Crown did not have to prove motive, she said there could not have been any reason why Younes would kill the child he had raised since he was one year old.

The Crown has highlighted three different versions given by Younes as to why he was in the home.

But Ms Rowan said "these statements have nothing to do with the death of Steven" and he had been entirely consistent about what happened from the moment he found the teenager.

She referred to Younes' voice on the triple-zero call, saying as well as being an alleged criminal mastermind he must have been an "extraordinary actor".

"That was distraught, that was the voice of someone absolutely devastated by what he had come across."

Ms Rowan also said she didn't have to prove Steven's epilepsy played a part in his death, it was up to the Crown to prove it didn't beyond a reasonable doubt.

She referred to evidence about an overseas study relating to 482 drowning deaths, in which 25 were directly related to seizures while the person was having a bath while unsupervised.

The trial continues before Acting Justice Peter Hidden.

Australian Associated Press