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"Mum was on the treadmill and got off and turned it off. Then she fell over and hit her head and went to sleep, so I went to sleep with her," Astin told his father.
Fietje spoke of the pain of hearing of the last moments of the mother of his two children: Astin, who turns 4 today, and Axell, 1.
"It was so hard to hear this from Astin. I cannot comprehend why and am in complete disbelief this happened," Fietje said.
Last Saturday, the South Auckland family held the funeral for 29-year-old Shari Fietje, whose death is yet unexplained, at Pukekohe Reformed Church.
Fietje shared the news of his wife's death on December 3 in a Facebook post.
"On Tuesday I lost the love of my life, someone so special and so dear to me, such a great mother, wife and friend to many," Fietje wrote.
"Shari was getting back into running as it had been a while as she had her nose reconstructed again, so with that healing up she must have decided Tuesday was the day to start back up and go for a run."
Shari's accident occurred in the shed of their Ararimu property, with Astin watching his mother run, and toddler Axell asleep in the house.
"It was so hard to hear this [account of Shari's fall] from Astin, I am not sure of the exact cause yet but nevertheless it caused Shari to pass away at only 29," Fietje said.
Police said they were aware of the death and had referred it to the coroner.
Fietje this week told the Herald on Sunday Shari's death likely wasn't the result of her tripping and hitting her head.
"We still don't know how it happened and are waiting on the coroner, but it's believed Shari had a medical event which resulted in the fall," Fietje said.
In his online tribute to his wife, Fietje remembers his wife's passion for running - posting a picture of her after she completed a 10km marathon in Tauranga.
"I was proud of her then, but I really wish I expressed how proud I feel now. This marathon was her first of many to come, and with tears I write was her last.
"I can only look to God and trust that this was in his time and I find some comfort in that."
The fracture to their family life is also already starkly evident to Fietje.
"Shari was such a great mum, she absolutely loved our boys Astin and Axell, and loved where we lived, our first home together. It's our slice of paradise," he wrote.
"We would always talk about our plans for the house and what we would do next. Something I am going to find harder than ever now.
"I am so sorry Shari won't get to see Astin and Axell grow and develop, or share any more laughs and adventures with us. The thought of this I cannot comprehend, it's just not fair.
"You are still in all of our hearts."
Fietje also anticipates the difficulty of explaining to his two young sons their mother's absence when they are old enough to fully understand.
"The boys are doing really well, but I know as their understanding develops, there will be a lot of questions and it's going to be so tough seeing them grow up without a mum," Fietje said.
"Treasure every moment you have with your wives, as you just don't know when it can all be taken away."