He said Callaghan and his new fiance had moved into the leafy suburb of Remuera so their son could go to a high-decile primary school.
Mr Grieve said he would not go into detail but Ms Thomas was a "troubled person'' and on the day she died she had called Callaghan over to her place to discuss their son's schooling.
He said Ms Thomas told Callaghan their son was being bullied at school and should be moved. She also told him: "You don't have any say in this because you're not his father.''
Mr Grieve said Callaghan's reaction was "well out of proportion'' but it provided some level of understanding of what had happened.
Crown prosecutor Simon Moore SC said the attack was "sustained and forceful'' and made worse by the fact it occurred inside Ms Thomas' home, where she had a right to feel safe.
"Ms Thomas was struck to the head with at least eight blows using a weapon we now know is a child's baseball bat.''
He said Callaghan's attack caused Ms Thomas' skull to be fractured. He said partway through the attack there was a "lull'' and a neighbour heard Ms Thomas scream before hearing a man's voice say: "shh... shh, its okay, be quiet.''
"Despite that,'' Mr Moore said, "Ms Thomas was hit again to the back on the head.''
He said it was the second set of blows that finally killed her.
Mr Moore said despite Callaghan being a loving father to his son, he had used the boy "as an artifice'' to suggest Ms Thomas was still alive.
He sent text messages to himself from Ms Thomas' cellphone which said he had to look after their son while she left Auckland, Mr Moore said.
One of the text messages from Callahan to Ms Thomas said: "Can you please call. Jack wants to talk to you. Also, want confirmation of Sunday. Hope you're better.''
Mr Moore said: "By this time, the prisoner had already dismembered and buried Ms Thomas.''
He said Callaghan made 12 calls to Ms Thomas' phone before finally going to police to tell them she was missing.
"Jack himself was a pawn in the strategy to avoid detection.''
He said Callaghan lied to Ms Thomas' neighbour shortly after the murder, recruited friends to help him get rid of the body and clean Ms Thomas' flat, bought a sim card in a fake name so he could call his friends undetected and texted Ms Thomas' friends from her cellphone to make it look like she was still alive.
"This lengthy catalogue demonstrates the sophistication of the planning,'' Mr Moore said.
The court also heard from Ms Thomas' mother Theresa Scott, who told of the pain and hurt in her life following her daughter's brutal murder.
Ms Scott said she had lost touch with her daughter and five year-old grandson but they were trying to reconnect at the time of the murder.
"Jack had to suffer the loss of his mother and has been told his father did this. When we chat on the phone, he sometimes sounds very down and upset.''
While trying to hold back tears, she said one can only imagine what is going through his "little head''.
Ms Scott said she has suffered from sleeplessness and loss of concentration at work.
Justice Geoffrey Venning said Callaghan had not been "emotionally equipped'' to deal with the problems Ms Thomas had raised about Jack's school.
He said nothing Ms Thomas told Callaghan on the day of the murder could excuse his response.
In his sentencing, Justice Venning took into account Callaghan's early guilty plea, his previous clean record and his remorse.
"In his written submissions Mr Grieve has said this is a tragic case. The words 'tragic' and 'tragedy' can be overused but I agree your case can be described as tragic.''
He said the death had affected Ms Thomas' family, Jack and Callaghan's new partner and their child.
Outside court, the officer in charge of the investigation, Detective Inspector Mark Benefield said the case had a devestating effect on the Thomas and Callaghan families.
He encouraged anyone in a violent relationship to seek help.
Family grateful
The immediate family of murdered woman Carmen Thomas are grateful at the 13-year sentence handed down to her killer.
However, other members of the extended family have slammed the penalty as "disgusting" and unjust.
A statement issued by Thomas' mother, step-father and sister said they were satisfied with the actions of police and the New Zealand justice system.
Teresa, Wayne and Storm Scott said the sentence was a milestone.
"We thank God that we have reached this point today but acknowledge that nothing can ever bring Carmen back to us and for that, we are angry.
"We want to express our sincere gratitude and thanks to the police who acted professionally and compassionately throughout the search for Carmen and over the ensuing several months.
"We are grateful to see that the New Zealand justice system works the way it does and that Carmen's killer will be out of circulation for a long time so that he might be able to reflect on the immense damage he has done to so many more lives than he can ever imagine."
However, other members of the extended family have expressed anger and disgust at the sentence on Facebook.
Thomas' aunt Carol Lavagna said the sentence did not reflect the "absolutely shocking" time Thomas' immediate family had endured since the murder.
"No one person should have to go through what you have gone through and I am absolutely disgusted in the outcome. All the family here in the UK are thinking of you 100 per cent and I know by hook or by crook all of us together will get through this. Love you all."
Cousin Craig Lavagna said Callaghan deserved to be given life in prison or the death penalty.
"Justice where was that?
"That is so wrong. The system there in s**t. Sorry cuz. Love to you... Carnt stay on here it makes me sick thinking he got off so lightly."
Others backed the family, saying Callaghan should have been given a tougher penalty for killing an "amazing woman"
Nikki Civin said she could not believe Callaghan "got off so lightly".
"Disgusted with the sentence. My heart goes out to your family."
Lisa Whyte said: "Sorry you didn't get enough justice for Carmen. RIP."
Nikki Tremayne said the sentence did not reflect the devastating impact Callaghan's actions had on Thomas and her family.
"Not alot for destroying ur sons life & taking an amazing women away!"
Kirstin Dufty said: "I wish this case was heard in America...a life for a life. Heartbreaking news."
Callaghan used 'significant force'
The 33 year-old appeared for sentence at the High Court at Auckland today dressed in a grey suit and flanked by security guards.
Crown prosecutor Simon Moore SC said Callaghan used "signficant force".
"Ms Thomas was struck to the head with at least eight blows using a weapon we now know is a child's baseball bat."
He said during the attack, Ms Thomas screamed and a neighbour heard a man say: "shh... shh, its okay, be quiet".
Mr Moore said Ms Thomas was struck again on the back of the head with what proved to be the fatal blow.
He said that showed there was a "lull" in the attack, which happened in Ms Thomas' home where she had a right to feel safe.
Details of the fatal argument have been heard for the first time today.
Callaghan's lawyer Stuart Grieve QC said his client was called to Ms Thomas' house to talk about their then five-year-old son being bullied at school.
The court also heard how Callaghan and his new fiance had moved to Auckland's Remuera so Callaghan's son could go to primary school in the high-decile suburb.
"The deceased said to Mr Callaghan: `You don't have any say in this because you're not his father'."
Earlier the court heard from Ms Thomas' mother. She told the court of the pain and hurt in her life after her daughter's brutal murder.
Theresa Scott told the court that she had lost touch with her daughter and five year-old grandson but they were trying to re-connect at the time of the murder.
"(The five-year-old) had to suffer the loss of his mother and has been told his father did this. When we chat on the phone, he sometimes sounds very down and upset."
She said one can only imagine what is going through his "little head".
Ms Scott said she has suffered from sleeplessness and loss of concentration at work.
Her family has spent about $26,500 on travel and legal fees.
Callaghan was arrested in September 2010 after a three-month investigation into the disappearance of Ms Thomas, 32.
Police found her remains in plastic containers buried in a shallow grave in bush off a walking track in the Waitakere Ranges.
A post-mortem examination showed she died from multiple blows to the head on the morning of June 29, 2010.
The fatal strike fractured her skull behind the right ear and the shock caused such severe damage to her brain stem it killed her.
The Carmen Thomas Case
2010
June 27: Last confirmed sighting of Carmen Thomas, at Greenlane Countdown about 7.10pm. Last day her bank accounts were touched
June 28: Message sent from her Facebook account at 8.14pm
July 1: Police start seeking CCTV images from central Auckland
July 3: Text sent from Ms Thomas' phone to her boss at a Pakuranga men's club
July 7 or 8: Her car believed to be driven from Auckland to Hamilton
July 13: Car towed and Thomas reported missing
Sept 1: Ms Thomas' wheelie bin found by police
Sept 20: Police put a camera down a hole at a worksite on Victoria St in central Auckland
Sept 21: Ms Thomas' ex-partner Brad Callaghan charged with her murder. Police begin searching Callaghan's home
Sept 23: Police begin searching an East Auckland property where one of Callaghan's friends lives. They seize a boat, believed to have been used in a failed attempt to dispose of Ms Thomas' body, for forensic testing
Sept 24-26: Police stop searching Callaghan's Remuera house and the East Auckland home of an associate
Sept 27: Callaghan's fiancee Tanith Butler gives birth to his new son
Sept 27-28: Police receive information that Ms Thomas is buried at a site in the Waitakere Ranges. ESR scientists map out the area and begin searching
Oct 1: Concrete containers holding at least some parts of Ms Thomas are excavated and the find is announced at a hastily called press conference
Oct 2: Police continue searching the site and expect a post-mortem examination will be carried out
2011
May 25: Callaghan pleads not guilty to murder at the High Court at Auckland
July 15: Court documents show police say Callaghan killed Ms Thomas with multiple blows to the head
Sept 17: Callaghan denied bail at the High Court in Auckland
Oct 13: Ms Thomas farewelled by family and friends at a service in Auckland
Nov 25: Callaghan pleads guilty to the murder of Ms Thomas
20112
Mar 30: Callaghan jailed 13 years and eight months for murdering Carmen Thomas.
