A Melbourne man who slashed the throat of a part-time drug dealer is scheduled to be sentenced for murder.

Josef Molfese was found guilty by a jury in June of murdering Tung "Ricky" Trinh, who died of "catastrophic" blood loss on March 18, 2016.

Mr Trinh's carotid arteries were almost severed in the South Melbourne attack, when Molfese attempted to rob him of drugs he thought he had.

Mr Trinh, 47, had been at a South Melbourne supermarket buying ice cream for his girlfriend when Molfese attacked him with a curved blade up to seven centimetres long.

His girlfriend Natasha Lamb, who was also injured, previously told the court she felt like she "was in a horror movie" during the attack.

She said "Ricky didn't say a word", with his body twitching as he lay dying.

Molfese, who has been described as having a "poly-substance addiction", particularly to heroin, is due to be sentenced in the Supreme Court of Victoria on Wednesday by Justice Jane Dixon.

© AAP 2018