Less than a week after her five-year-old son was brutally stabbed to death in their Sydney home, the boy's mum has spoken out in support of her jailed partner and taken aim at the state's mental health system.

"I still love my partner. He's a sick man, he's not himself right now, it's the drugs. We've been together for years. I need to be there to support him when no one has," the woman, who cannot be identified, told the Daily Telegraph.

"He loved our son more than words can express. He was the apple of his eye and he couldn't wait for the day he was born."

Read the full interview in the Daily Telegraph.

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The five-year-old died in Westmead Hospital on Friday morning after he was allegedly stabbed in his bed by his father at their Carlingford home in Sydney's northwest.

Four days after losing her son, the boy's mother, who works as a hospital nurse, said his death was "the price I'm having to pay" for the apparent lack of help her partner received for mental health issues.

It's understood the 36-year-old father was repeatedly turned away from mental health facilities in the week leading up to the alleged murder.

The father first tried to get help at a mental health facility in Castle Hill but instead had his medication doubled, according to 9 News.

He then went to another facility in Hornsby and asked for help but was told there weren't any spare beds. On a third attempt, he returned to the same Castle Hill facility where he was again turned away and given more medication.

Finally, the day before he allegedly stabbed his son to death, the father was waiting on a mental health team to visit the family's home — but they never showed up.

The father had also recently spent time in a mental health facility and had been forced to take time off work due to mental health issues.

The mother also claimed she had been asking the system for help for "some years" before her partner allegedly stabbed their son.

Early on Friday morning, the five-year-old's grandmother returned to the Carlingford home after dropping her daughter off at work to find her grandson with serious knife injuries.

Bundling him into her car, she attempted to find him medical help but eventually realised how serious her grandson's injuries were, the grandmother pulled up in front of a childcare centre and called for help. She was later treated for shock.

The mother's comments about being "bitterly disappointed" in the mental health system came two days after the jailed father fronted a Sydney court for the first time, appearing completely disoriented.

Wearing a green prison jumper, the 36-year-old father appeared in Parramatta Local Court on Saturday morning via video link and asked the magistrate "Where am I?"

The father, who is currently in Amber Laurel Correctional Centre in Emu Plains in the city's west, appeared to not understand what was happening and blankly stared at a wall as his matter was mentioned.

The boy's grandfather has also spoken out about the problems the accused was having before the alleged murder.

"There's a lot more to this," the boy's grandfather told 9 News.

The father was formally refused bail on Saturday and is due to front court again on July 30.

He had been questioned by detectives for most of Friday after allegedly stabbing his young son at Carlingford in Sydney's northwest between 6.30am and 7.15am.

The boy's grandmother tried to get him help by cradling him into a car and driving away.

She drove about 250 metres before she stopped to call for help, realising the extent of his injuries.

The five-year-old died about an hour after arriving at The Children's Hospital at Westmead.

"The level of violence is horrendous … it's probably about as bad as it gets," Superintendent Rob Critchlow told reporters on Friday.

Where to get help:

Lifeline: 0800 543 354 (available 24/7)
Suicide Crisis Helpline: 0508 828 865 (0508 TAUTOKO) (available 24/7)
Youthline: 0800 376 633
Kidsline: 0800 543 754 (available 24/7)
Whatsup: 0800 942 8787 (1pm to 11pm)
Depression helpline: 0800 111 757 (available 24/7)
Rainbow Youth: (09) 376 4155
Samaritans 0800 726 666
If it is an emergency and you feel like you or someone else is at risk, call 111.