Callaghan told his friend, referred to as Witness 70, he needed help to dispose of the body at sea. The two loaded the plastic containers into a boat, but Witness 70 later changed his mind and refused to help. But he did not call the police.
Callaghan was arrested on September 22 last year and police hinted that charges could also be laid against those who helped him.
The Weekend Herald understands police considered charging Witness 70 as an accessory to murder after the fact. A police spokeswoman confirmed prosecution immunity was granted to one witness, but declined to comment further.
No charges were laid against a second friend - who also has name suppression - who helped Callaghan after the murder.
Callaghan sent the man a text message which read: "Can you please bring 3 black rubbish bags and a bottle me kerosene [sic] ... keep it quiet."
The friend bought a bottle of kerosene, then met Callaghan at a supermarket, where he bought large plastic bags and cleaning products. A week later, he also picked up Callaghan after he left Ms Thomas' car in Hamilton to throw the police off the trail.
After Callaghan was arrested, police raided the Auckland man's home and seized his boat.
But no charges were laid as there was no evidence to suggest he knew Callaghan had killed Ms Thomas.
She and Callaghan had separated in 2005 after the birth of their son, Jack, and shared custody.
Callaghan was to stand trial next February and news of his shock guilty plea was greeted with joy by friends, family and workmates of Ms Thomas.
Her aunt Carol Lavagna posted a message on a Facebook tribute page saying "brilliant news at last", and friend Sandra Gurney said: "He's finally done something right."
New court documents released after the High Court hearing revealed that Ms Thomas suffered multiple blows to her face and head at her Remuera home on the morning of June 29 last year.
Callaghan "embarked on a sustained course of conduct to cover up" Ms Thomas' death and deflect police attention away from himself, the documents said.
After a three-month police investigation, Ms Thomas was found buried in a shallow grave in the Waitakere Ranges, but police still do not know where she was dismembered.
Outside court yesterday, the officer in charge of the investigation, Detective Inspector Mark Benefield, said investigators had worked hard to solve the case.
It was a "pleasing day" for Ms Thomas' family and the members of the police team.
Police had empathy for the victims, he said.
"We have a 5-year-old that's lost his father and mother. We have a mother that's lost her son. We have a mother that's lost her daughter. I'm a father ... Prick me, do I not bleed? I'm human."
Callaghan will be sentenced in March.
