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

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

While Impson admitted killing Mr Insley, her lawyer unsuccessfully contended she acted in self-defence and was substantially impaired due to her bipolar affective disorder and borderline personality disorder.

In jailing her in the Supreme Court on Friday, Justice Michael Walton found there had been "a modest degree of provocation" by Mr Insley towards Impson who had armed herself with the knife when he turned up unexpectedly.

She was in a "heightened emotional state" - not only due to the death of her cat Angel but also because of her mental health disorders.

Read more: Windang island murder: killer drove knife in 14cm

Outside court, the victim's sister, Linda Reynolds, said the family was happy with the sentence although "no amount of time will ever be enough ... eventually she will get out".

She did not believe "one word" of an apology letter written by Impson.

Impson and Mr Insley, who first met about seven weeks before he was killed, had some run-ins in the lead-up to the stabbing during which she had asked bystanders to call the police.

Impson also received a call from the RSPCA shelter saying her cat had been found run over.

She sent Mr Insley a text saying: "I know you killed Angel, watch your back c***."

At her sentence hearing in August, Impson denied she'd killed her boyfriend in anger as she suspected he was involved in Angel's death.

Rather, she said she'd been scared and afraid when he turned up outside her tent.

Justice Walton said Impson had a demonstrated history of mental illness, was unlikely to reoffend and had reasonably good prospects of rehabilitation.

"I consider the offence to be in the low range of objection seriousness," he said.

He set a maximum term of 18 years in jail.

Australian Associated Press