During the visit McIsaac broke into a property in the small township and removed bedding and a wood splitter.
After buying a drink at a local shop, the pair went to the beach, where Alex fell asleep in the bedding.
"After Alex fell asleep you struck him twice in the head with a wood splitter. Alex would have died immediately and wouldn't have known what happened," Justice Collins said.
Alex's body was left on the beach. The next day McIsaac called his mother from a phone box and said he was distressed and needed to talk.
The pair walked around the beach but McIsaac did not tell the truth about what happened or help police with their ongoing search for Alex.
"Your conduct is impossible to comprehend, particularly given the close relationship you had with Alex," Justice Collins said.
Mrs Fisher's victim impact statement said the whole family was struggling with what happened.
"Your offending is really the worst nightmare for a parent," Justice Collins told McIsaac.
The Crown requested a minimum jail term of 17 years. Justice Collins made reductions for a prompt guilty plea and McIsaac's "significant" mental health impairments.
The judge said McIsaac's lifelong battle with mental health problems escalated before he was jailed for burning his father's flat.
Before the killing, McIsaac believed he had terminal cancer and Alex was being abused.
The defence argued Alex wasn't necessarily vulnerable because of his age, but the judge thought otherwise.
"He was just 10 years old," he told McIsaac. "He was asleep at the time you murdered him. He had no chance of fleeing or defending himself."
Justice Collins thought there was a "degree of planning and pre-meditation".
McIsaac stole the weapon and bedding and waited for Alex to fall asleep before striking him. He asked his mother to bring Alex to the beach.
McIsaac wore prison-issue clothing and was impassive in the dock. Justice Collins ruled no photographs of him could be taken today.
He admitted murdering Alex at Waitarere Beach on October 5 or 6 last year.
Alex's body was found at the beach on October 8 after his disappearance sparked a huge three-day manhunt.
During the search, police released CCTV footage showing Alex and McIsaac together in a local store on October 5.
McIsaac appeared in court on October 7, initially charged only with possessing a knife in a public place.
Crown prosecutor Ben Vanderkolk said he wouldn't go through the facts of the case because everyone who needed to be familiar with them already was.
There was a "degree of brutality" in the case, Vanderkolk said.
"It was a blow with a heavy blunt instrument."
The killing was inexplicable and the motive unknown because McIsaac still refused to talk about what happened.
Vanderkolk asked for a non-parole period of 17 years.
He referred to Mrs Sandra Fisher's victim impact statement which was written with "considerable dignity" and in a "measured way".
"It is impossible to understand the profound grief that she must feel, has felt and will continue to feel," Vanderkolk said.
However, her maternal instincts extended to the care of her surviving son.
"She urges upon the court a merciful outcome."
Vanderkolk said if McIsaac received the right treatment in the right place, Alex's death might not be in vain.
Defence lawyer Letizea Ord acknowledged the trauma McIsaac caused to his family and said his mother had effectively lost two children.
The two siblings shared a good relationship and enjoyed spending time together.
"That's what makes this case so difficult."
Ord said there was no doubt the "underlying mental health" problems, present for many years, were behind McIsaac's actions.
Problems were confirmed in seven psychiatric reports and by family and friends.
But when McIsaac was first spoken to by psychiatrists, he didn't want to "plead insanity" and was prepared to face what he'd done.
Ord said the killing wasn't calculated or pre-meditated or committed with a high level of cruelty, as legislation characterises the worst murders.
Statement from Detective Senior Sergeant Neil Forlong:
Police acknowledge the sentencing today of Eric McIsaac in relation to the murder of his half-brother Alex Fisher.
While we know this is an important step for Alex's family, this is no closure for them as they continue to grieve.
Police would like to thank the family for their patience, understanding and above all dignity while we worked with them through the difficult steps of an investigation and court process.
I would also like to thank the police team who worked diligently through the search operation and subsequent inquiry.
We know that there are still unanswered questions about the exact circumstances of Alex's death.
The only person with the answers is Eric McIsaac, and he has yet to share them with us.
Finally it is important that we thank the wider community for its support to Police and Alex's family since October last year.
While this has been a tragic and shocking event, it is has also brought out the best in our community here in Horowhenua.
Statement from family of Alex Fisher:
We would like to thank the people of New Zealand and the Horowhenua community for their love and support throughout this tragic time.
We ask to be left alone by the media. Our Family and friends, and the Horowhenua community, need time away from the public eye to grieve.
We may release a statement in time to come if and when we are ready.
Alex was much more than the sad events surrounding the end of his life.
We his parents, siblings and family had nearly 11 years of memories of a treasured and much loved boy who will forever live in the hearts of so many.
We wish to focus on these memories and do the one thing that matters the most - REMEMBER ALEX.
