David John Campbell, 45, had no idea the victims Genine Ballantine and Steven White were inside the unit when he reached through a broken loungeroom window and set fire to a curtain.

Fuelled by alcohol and the drug ice, Campbell's plan was to burn the flat to the ground so Ms Ballantine would have to return to live with him, and Mr White would be homeless.

Ms Ballantine and Mr White, both 44, and their Jack Russell dog Rosco died in the fire on September 20, 2013.

Campbell pleaded guilty in the Supreme Court on Wednesday to two counts of arson causing death. The maximum penalty for each charge is 25 years' jail.

Crown prosecutor Anne Hassan said Campbell's actions had been reckless and grossly negligent, but the prosecution accepted he did not intend to kill Ms Ballantine and Mr White.

Ms Hassan said the fire was the final act in a relationship based around drug and alcohol abuse, and prolonged domestic violence.

Ms Ballantine had taken out 11 intervention orders against Mr Campbell during their almost 20-year relationship, while he had taken out two against her.

Ms Ballantine had left Mr Campbell in June 2013 and begun a relationship with Mr White, which also became violent.

After the fire quickly engulfed the unit, Mr White, a father of three, yelled out to two housemates, "The house is on fire, get out," the court heard.

The housemates managed to escape but could hear Mr White and Ms Ballantine calling out to each other and screaming.

In a victim impact statement read to the court, Mr White's mother Brenda said all she wanted to do was go to sleep each night so she could see her son in her dreams.

She said she missed his laugh, his cuddles and most of all him saying, "I love you, mum."

Defence barrister Dermot Dann said Campbell had a violent upbringing, during which he was regularly beaten by his alcoholic stepfather, who introduced him to drugs and alcohol from an early age.

Mr Dann said Campbell, who had met Ms Ballantine in 1994, could not live with her and could not live without her.

Campbell told police Ms Ballantine had been the love of his life and that he had been grief-stricken the day after the fire when told she had perished.

He claimed the last thing he had wanted to do was hurt Ms Ballantine despite the years of domestic violence, and that he deeply regretted lighting the fire.

Justice Lex Lasry will sentence Campbell on April 24.