Just after midnight on June 13 last year, Todd attacked the 22-year-old in Princes Park in Carlton North, then raped and strangled her.

He later returned to the scene of the crime but was ushered away by police.

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Todd has since admitted he murdered, raped and sexually assaulted Ms Dixon.

Professor Ogloff, who has spent time examining Todd, also refuted earlier suggestions that the killer’s autism spectrum disorder diagnosis had contributed to his serious offending.

He said while it could go a way to explain the young man’s struggle to express or feel some emotions, Todd showed he was fully aware of his surroundings during the 54 minutes he trailed Ms Dixon from Flinders Street Station.

Todd also admitted he knew his victim's ‘‘level of fear’’ during the attack.

‘‘He was glad he was caught because the event has scared him,’’ Professor Ogloff said.

The forensic psychologist also wanted to make it clear that the vast majority of people on the autism spectrum don’t display violence.

He said the biggest contributing factor to Todd’s actions were his diagnosed sexual sadism disorder which research showed was present in up to 40 per cent of sexual homicides.

Todd had begun watching pornography at 13 and in the year leading up to Ms Dixon’s murder, the online content had turned increasingly violent.

Professor Ogloff labelled Todd’s sex disorder as ‘‘profound’’.

‘‘He’s now carried out the darkest fantasy,’’ he said.

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When asked about Todd’s prospects of rehabilitation, the professor said research showed there was no clear evidence that long-term medication could eliminate the disorder.

‘‘He has been exposed to these thoughts for at least a fifth of his life.’’

The plea hearing continues.