Arun Kamalasanan and Sofia Sam were jailed for 27 years and 22 years, respectively, after a jury found them guilty of murdering 33-year-old Sam Abraham at his Epping family home in October 2015.

Kamalasanan was the architect and driving force of the poison plot, but the Court of Appeal on Friday found his sentence to be disproportionate compared with Sam's.

"The verdict exposed Sam's role as a full and willing participant in this serious premeditated crime," the appeal judges said.

"It was not open to (Justice Paul Coghlan) to distinguish between Kamalasanan and Sam to the extent that he did."

Kamalasanan was re-sentenced to 24 years' jail, with his non-parole period reduced from 23 to 20 years.

But the court threw out bids by Kamalasanan and Sam to appeal their convictions.

Sam had claimed the jury's guilty verdict was unsafe and unsatisfactory, and that she should have been tried separately to Kamalasanan.

The appeal judges didn't agree and said the overwhelming evidence from the trial meant it was unrealistic for the jury to return anything but a guilty verdict.

Kamalasanan's arguments against his conviction, including claims of new evidence he said could help clear his name, were found to be "devoid of merit".

Mr Abraham was found dead in his pyjamas and foaming at the mouth. He was initially thought to have suffered a heart attack before an autopsy pointed to cyanide.

Detectives spied on Kamalasanan and Sam for months and found a secret diary they shared.

"Can you hold me tight? I want to drift away in your love," one of Sam's entries read.

"She is the best match for me - but what to do? I am sure that one day she will be mine," Kamalasanan wrote.

The jury was told there were two possible ways Mr Abraham could have been poisoned.

One involved Sam and Kamalasanan sedating him before pouring cyanide-laced orange juice into his mouth.

An alternative was was that Sam prepared a cyanide-laced drink for Mr Abraham before he went to bed.

Australian Associated Press