A young man strangled his mother to death before pouring petrol over her and setting her alight in her Auckland home, a court has heard.

Kiri Paul Patira Brackenridge is on trial before a jury in the High Court at Auckland accused of murdering Linda Winifred Edwards.

The 58-year-old was found dead inside a Rainford St, Mt Roskill home after firefighters fought a blaze there on November 8, 2016.

Brackenridge, 28, is defending the charge with a not guilty by reason of insanity plea.

He inadvertently pleaded guilty to murder this morning before correcting his answer to not guilty. Brackenridge is also charged with arson and dishonestly using a document.

Crown prosecutor Scott McColgan told the jury Brackenridge strangled his mother to death some time between 10.30pm and 11.30pm on November 8.

He then left her body on the couch and drove away in her car with her credit card.

Brackenridge then stopped at an ATM machine, McColgan said, before buying a packet of cigarettes and $1 worth of petrol.

Upon returning to the house he poured the petrol over his mother's lifeless body and set her alight.

"None of it is in dispute in this trial," McColgan said.

"There is no dispute about what Kiri Brackenridge did in this trial ... He killed her and he meant to kill her."

McColgan said the trial was not about what happened, but more about why.

"This is a trial about whether Mr Brackenridge was legally insane at the time he killed his mother."

He said there was no doubt Brackenridge was psychotic when he killed his mother, but this was because of his methamphetamine use rather than having a "disease of the mind".

Marie Dyhrberg QC, who is defending Brackenridge, said her client accepts he killed his mother, bought the petrol and set fire to the house.

However, the veteran defence lawyer said despite Brackenridge's meth use he was legally insane at the time Edwards was killed.

Justice Pheroze Jagose earlier declined the Herald's application to photograph Brackenridge during the trial, which is expected to last two weeks.