Walcha murder trial: Judge warns jury of assisted suicide plea in Natasha Beth Darcy's trial for murdering Mathew Dunbar in Walcha, 2017

Justice Julia Lonergan has begun her summing up of the nine-week murder trial for Natasha Beth Darcy, who stands accused of murdering her de-facto partner, sheep grazier Mathew Dunbar, on August 2, 2017.

Darcy, 46, has pleaded not guilty to murder but at the start of her trial offered a plea of guilty to aiding or abetting suicide, which was rejected by the Crown.

Late on Thursday afternoon Justice Lonergan said the defence had not suggested "that you would or could bring in such a verdict" in its closing statements.

"Assisted suicide should be put out of your mind entirely," Justice Lonergan told the jury.

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She told the 11-person jury that "trials are unpredictable" and sometimes "a different approach is taken" based on the evidence given in the trial.

The legal direction came after a morning of closing submissions from Darcy's defence barrister, Janet Manuell SC.

The defence said Darcy had told "lots and lots of lies" but there were a dozen reasons to acquit Darcy of murdering her partner.

The 46-year-old denies giving the Walcha sheep grazier a drug-filled blended drink and then gassing him in bed before his death on August 2, 2017.

Prosecutor Brett Hatfield claims Darcy made dozens of searches from February 2017 on how to murder or poison her partner, in order to inherit his multi-million dollar property 'Pandora'.

Ms Manuell pointed to the two letters Darcy sent to a friend, which the Crown maintains included an offer to give payment if she lied for her in the trial.

Ms Manuell told the jury that "you know that people can do stupid, desperate things" when they feel "they've been unjustly accused".

She said the letters were written when Darcy was in custody and at a time when she "thinks she has been wrongly charged with the murder of Mr Dunbar".

Ms Manuell said "there is obviously evidence that Ms Darcy told a lot of lies".

"Ms Darcy has made things worse and worse for herself by the repeated lies she has told," she said.

The defence told the jury people lie for different reasons, she said some "fear they might get in trouble" or "feel like they've been unjustly accused", or "feel they're being pre-judged".

Ms Manuell said the Crown must rule out the reasonable possibility that Mr Dunbar "died by his own hand", and added, "we say there are at least 12 reasons why the Crown can't do that".

Ms Manuell said it included "what we know of Mr Dunbar's depression"; his reported suicidal ideations; the suicide searches on the computer as well as one on how to stop suicidal thoughts.

She told the jury the Crown could not rule out that Mr Dunbar "was an active and knowing participant" in the purchase of the helium bottle.

Ms Manuell also said it was implausible that Darcy watched an exit bag video not once but twice while sitting at a Peel Street cafe, next to Mr Dunbar the day before he died.

She said it was further compounded because the helium tank had already been purchased prior to the cafe visit.

The Crown claims Darcy blended a cocktail of drugs and sedatives in a Nutribullet and fed it to Mr Dunbar at dinner on the night of his death, but Ms Manuell said it was also implausible that Mr Dunbar had drunk a drink "with all those crunchy bits".

"It is not for Ms Darcy to prove anything," she said.

The trial resumes on Friday.

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