'Irrational jealousy' behind Natthawut Tammajanta's brutal choking murder of his partner Manthana Khantharat after she had locked herself in the bathroom of their Albion Park Rail home

A man who murdered his partner after she tried to hide in a bathroom has been sentenced to more than two decades in jail.

Manthana Khantharat hid in a bathroom in an attempt to escape an intense and fatal assault by her jealous and controlling boyfriend, a court has heard.

The 37-year-old's killer, Natthawut Tammajanta, was on Wednesday sentenced to up to 23 years and four months in jail after pleading guilty to her murder.

The pair, both originally from Thailand , met while working at the Siam Kingdom restaurant in the Sydney suburb of Newington and started a relationship.

But after an argument on December 18, 2022, Tammajanta assaulted Ms Khantharat at their home in Albion Park Rail, south of Wollongong, so severely that he killed her.

The attack included choking and striking her repeatedly to the face and body.

In her sentencing remarks, Justice Natalie Adams said police who arrived at the scene observed Tammajanta's hands to be lacerated and swollen from his repeated assaults on Ms Khantharat.

'At one stage, the deceased had locked herself in the bathroom to escape the assault, but the offender was able to break down the door to continue his attack,' she told the NSW Supreme Court .

Tammajanta described himself as a 'jealous person' who felt insecure in the relationship and was suspicious that his partner was dishonest or too close with other men.

'The deceased was killed in her own home by an intimate partner,' Justice Adams said.

'The dominant feature of the relatively short relationship was the offender's intense irrational jealousy and his desire to control the deceased's behaviour.'

Earlier the court had heard from Ms Khantharat's younger sister, Rudeemas Khuntaras, who spoke on behalf of the family.

'The sudden death has been devastating,' Ms Khuntaras told the court, reported the Illawarra Star .

'She was a huge part of our lives. We are all struggling with the intense grief and we are all struggling to deal with her being gone.

'She was the older sister of three, she was an exceptional sister and she was a gatekeeper who provided us protection.'

Ms Khuntaras spoke of her life-changing grief after she'd been informed by police of the discovery of her sister's body.

'I couldn't understand what had happened,' she said.

'It is the hardest thing I've ever done in my life. I had to call my mother in Thailand and tell her her daughter was dead.

'I was crying every day for months ... we miss her every day and I don't think that feeling will ever go away.'

Tammajanta was given a non-parole period of 16 years and four months.

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