The 25-year-old man, who cannot be named because that could identify the victim, pleaded guilty in Warrnambool County Court on Thursday to making a threat to inflict serious injury, sexual assault and two counts of common law assault.

The man was expected to stand trial on Wednesday but the matter resolved to a guilty plea early in the day.

On Thursday the court heard the man and the then 23-year-old victim were in a long-time relationship and at the time of the offending were living together in the Southern Grampians area.

The man was a methamphetamine user and the pair regularly argued about his drug use.

Then in January 2020, the victim returned home from work and the pair argued over her refusing to smoke the drug ice with him.

When she attempted to leave, the man grabbed her by the arm and threw her into a wall, causing her to fall to the ground and become visibly upset.

He threatened to slit her throat before slapping her to the face five times.

The man then sexually assaulted the woman, causing her to hyperventilate and suffer from a panic attack.

The victim was left "petrified", court documents obtained by The Standard showed.

The court heard she returned home from work again two days later and the man sounded scared, was talking about volcanoes erupting and wasn't making sense.

He told her to pack her things, an argument ensued and he threw a large knife in her direction.

It hit a wall and left her in fear, the court was told.

The man presented at a south-west hospital's emergency department later that week.

He appeared panicked, scared and reported suffering from auditory hallucinations.

The man believed people were spying on him, following him and wanted to kill him.

He told a doctor he hadn't seen the victim for a while and he feared "something bad had happened to her".

The man said the last thing he remembered was consuming a cocktail of methamphetamine, cocaine, cannabis and alcohol.

Medical staff believed the man was likely suffering a drug-induced psychosis.

He was later medically discharged and subsequently arrested at a Hamilton motel.

The man has no criminal history.

He will be psychologically assessed and will appear in Melbourne County Court for a further plea hearing on November 7.

He is expected to give evidence on that day.

If you or someone you know is impacted by sexual assault, domestic or family violence, call 1800RESPECT or visit 1800RESPECT.org.au. In an emergency, call 000.

Call Lifeline 13 11 14.

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