For almost two years a Sydney couple got away with feeding their baby a strict vegan diet that left her so malnourished she looked like a three-month old — but in the end it was their lies that brought them undone.

On Thursday, the 33-year-old mother and 35-year-old father were sentenced to an 18 month intensive corrections order for their crimes and more details emerged of the horrific impact their actions will forever have on their daughter's life.

The 20-month-old girl was kept on a strict vegan diet of oats, bread and a few mouthfuls of vegetables, had never seen a doctor and was unvaccinated. news.com.au reports.

When the child was taken into care she weighed just 4.89kg, less than half of the average weight of a child for her age.

It took the child having a fit and her lips turning blue for her parents to finally seek medical help.

She was rushed to hospital where she was examined by a doctor for the first time since leaving the hospital as a newborn.

The girl's poor health and stunted development was enough to spark an investigation.

Despite clear signs their child was severely malnourished, the parents "were initially unable to accept that their daughter's condition was due to malnutrition", Judge Huggett said in court on Thursday.

The parents even allegedly tried to cover up the neglect by lying to authorities about their daughter's health and development.

The judge blasted the "inconsistent and untruthful" statements they gave about the girl's "developmental milestones".

The parents also lied about the girl's diet, telling investigators she had occasionally eaten animal products, would soon be vaccinated, had regularly seen doctors and had been growing.

At 20-months-old most babies have started to speak, have teeth and can crawl or even walk.

When the child was hospitalised she couldn't speak, had no teeth and was unable to crawl or even sit up on her own.

She had little muscle tone, low blood sugar and calcium levels and was suffering from rickets, a preventable bone disease that had caused minor fractures.

Her restrictive diet stunted her growth and significantly delayed her motor skills.

Her development was so impacted that she qualified for the National Disability Insurance Scheme.

The foster carer who looked after the child when she was removed from her parent's care revealed the trauma the child was continuing to experience because of what her parents did.

"She was being fed through a tube in her nose," the carer said in a victim impact statement in May. "I remember thinking, how terrifying this must be for such a small child.

"I was also shocked by how far behind (she) was compared to other children her age I had looked after — they had been able to run around, talk to you, play games.

"Caring for (her) was caring for a very young baby. She couldn't sit up, she couldn't speak any words, she couldn't feed herself or hold a bottle, she couldn't play with toys.

"She spent the day in her cot rolling back and forth. She couldn't roll over all the way."

In January when the girl was about two and a half, she was only 76cm tall — the height for size zero clothing for a standard one-year-old.

The girl had to go to weekly occupational therapy appointments and monthly appointments for physiotherapy, speech therapy, and to see a paediatrician and dietitian.

She described watching the child having to be held down during her monthly and bimonthly blood tests, which left the young girl "traumatised".

Even after it became undeniable that the child's incredibly poor health was a direct result of her diet, the parents continued to make excuses.

Court documents showed the mother told doctors her daughter would generally have one cup of oats with rice milk and half a banana in the morning, and a piece of toast with jam or peanut butter for lunch.

She claimed the girl would also be offered tofu, rice or potatoes at dinner time but she was a "fussy eater", so she would likely just have oats again.

This diet resulted in severe deficiencies in nutrients across the board for the infant, including a lack of calcium, phosphate, vitamin B12, vitamin A, iron and zinc.

Her levels of vitamin D, which can cause bone disease if found to be too low, were "undetectable".

Judge Huggett repeatedly cut down the couple's excuses, both the ones they offered to investigators and the ones given in court.

Though she did accept the mother was suffering from a level of depression at the time, she noted neither parent was "very young or uneducated" and had two older children, so understood how to care for their daughter.

Their boys, aged six and four, were also on vegan diets and were taken into government care along with their sister.

During the trial the father said the mother was the main carer for the child, as his job meant he was not around often to oversee her care.

It was claimed the mother had become "increasingly fixated" on the family's vegan diet, and that the father would secretly eat meat outside of the home.

A solicitor for the dad suggested his client was "powerless" to prevent his daughter from falling ill.

His defence barrister, Frank Coyne, had previously argued the mother "dictated" the household, and claimed his client was the sole provider and would do all the driving, shopping, cooking and cleaning.

This argument didn't hold up very well with Crown prosecutor Julia Dewhurst, who pointed out the father "lied to hospital staff" about the child's development and he made a "conscious decision" not to vaccinate her.

She said that while he drove his children to school, he sent them there with "two pieces of bread and an apple."

"He cannot now claim the decisions were solely made by (the girl's mother)," she said.

This sentiment was echoed by Judge Huggett in her summation on Thursday.

"He had ample opportunity to observe his daughter's condition and he did nothing at all to address it, such as seeking help from other family members or seeing a doctor who he himself had seen," she said.

"The further abandonment of parental responsibility cannot be excused."

The court also previously heard the relationship between the couple had broken down over suspicions of an affair.

They have since separated and have been taken parenting courses since their arrest.

The mother has sought support for natal and postnatal depression which she said made her feel worthless, apathetic and listless.

Judge Huggett found both parents were equally culpable for what happened to their child, but accepted they were remorseful for their actions and there was a low risk of reoffending.

They were sentenced to 18 months imprisonment but won't have to spend time behind bars due to the sentence being in the form of an intensive correction order.

The judge found that a custodial sentence would deprive the three children of the "important bond" with their parents.

The girl and her brothers are now in the care of a relative. When the toddler was placed on a normal diet and given extra support she quickly began to grow and her teeth emerged.

She has since recovered from the malnutrition and weighs more than 12kg.

However, it is unclear how much of an impact her rough start to life will have on her as she grows, with her future developments remaining in doubt.