Defence barrister Ruth Shann said Michael Anthony O'Neill, 48, was receiving psychological treatment in prison but was having difficulties talking about aspects of the case "without becoming distressed".

O'Neill appeared briefly in the Supreme Court on Wednesday to formally plead guilty to one count of murder and one count of arson.

When asked what was his profession, O'Neill stood in the dock and said, "I worked in the interior design business".

Ms Shann said the defence needed time before O'Neill's pre-sentence hearing to take proper instructions from him and for a psychological report to be prepared.

Justice Elizabeth Hollingworth agreed to list the two-day hearing from December 15.

Mr Rattle's body was found in the couple's burnt-out South Yarra flat on the night of December 8 last year but police believe O'Neill killed his partner early on the morning of December 4.

O'Neill admitted taking a saucepan from the kitchen early that morning and striking Mr Rattle over the head before strangling him with a nylon dog lead.

Later, he covered the body with a large plastic bag and left it in the bed, and regularly sat there over the following days, crying and apologising. "[I] asked him to forgive me," he said.

O'Neill acknowledged in his police interview that he would have benefited financially from his partner's death.

- with Adam Cooper