A woman stabbed her boyfriend to death with a large knife after assuming he killed her cat, has been jailed for 12 years.

Rachel Impson, 42, was found guilty of murdering Michael 'Mick' Insley, 39, at Windang's Picnic Island in the Lake Illawarra region one night in October 2014.

He had turned up outside her tent, uninvited and without notice.

Rachel Impson has been jailed for 12 years after stabbing her boyfriend in the back with a large knife.

Impson's significant demonstration of mental illness and the death of her cat triggered her 'heightened emotional state'.

Her lawyer unsuccessfully contended her murder of Mr Insley was an act of self-defence. Instead, the act was due to her bipolar affective disorder and borderline personality disorder.

Impson admitted to killing Mr Insley and was jailed by Justice Michael Walton in the Supreme Court on Friday.

Mr Insley's sister Linda Reynolds, present outside the court, said the family was happy with the 12 year sentence although 'no amount of time will ever be enough ... eventually she will get out.'

Distressed at the murder of her own brother, she said she did not believe 'one word' of a written apology Impson had written for Reynolds.

Impson's murder was linked to her bipolar affective disorder and borderline personality disorder.

The victim's sister Linda Reynolds said the family is happy with the sentence but 'will never be enough'.

Impson and Mr Insley had only known each for for seven weeks before she executed the sudden murder and had run-ins in the lead-up to the stabbing where she asked bystanders to contact police.

When Impson received a call from the RSPCA shelter saying her cat was found dead, she immediately assumed Mr Insley had killed her pet.

'I know you killed Angel, watch your back c***,' she texted him.

Impson denied she murdered Mr Insley out of anger during her sentence hearing in August.

Instead, she justified her act by saying she was scared as he suddenly turned up outside her tent uninvited.

Justice Walton set a maximum term of 18 years in jail.