A toddler so badly starved by her vegan parents that she had no teeth and could not stand up is now obese - with her carer saying 'it's like she's storing calories in case she needs them in the future'.

The malnourished child, who was fed a diet relying on oats and rice milk, was 'defenceless and unable to protect herself' from her mother and father for the first 19 months of her life.

Details of the Australian toddler's neglect were revealed in Sydney's Downing Centre District Court on Thursday during a sentencing hearing for her parents.

The court has heard the child, now aged three, had no medical records after her birth and neighbours did not even know she existed.

Her mother and father, aged 32 and 35, fed the girl a diet so lacking in basic nutrients she developed rickets, a degenerative bone disease caused by a shortage of vital nutrients.

They have admitted to failing to provide for their child, causing her serious injury.

The father, 35, and mother, 32, of a toddler who was so malnourished she was suffering from rickets, a degenerative bone disease caused by a lack of vital nutrients

Victim: The baby who was so badly starved by her vegan parents that she had no teeth and could not stand up

For 19 months authorities had no idea of the neglect the toddler was going through until she had a seizure at the family's home in Sydney's eastern suburbs (pictured)

Neighbours told police they saw only the girl's two older brothers playing in their backyard and did not know the couple had a third child.

For 19 months authorities had no idea of the neglect the toddler was going through until she had a seizure at the family's home in Sydney's eastern suburbs.

A toddler was so malnourished by her parents she looked like she lived in the Third World

The girl stayed in hospital for a month in March last year with her mother constantly by her bedside.

A victim impact statement written by a foster carer on the child's behalf revealed the extent of her malnourishment.

'For the first 19 months of her her life (the girl) did not receive the basic care to grow and develop,' the carer said in her statement.

'(The girl) was defenceless and unable to protect herself from her parents.'

When the carer first encountered the child she was 19 months old but weighed less than 5kg (11lbs) and looked like a three-month-old.

She was shocked by how far the girl was behind in her development.

The child, who had no teeth, could not sit up or even roll over, let alone use her hands for simple tasks such as playing with toys.

'Caring for (the girl) was like caring for a very young baby,' the carer said.

The child, now almost three, had made progress but her height and weight were out of proportion.

Suffering: The toddler, pictured, developed rickets, a degenerative bone disease caused by a shortage of vital nutrients

The mother and father are pictured hugging as they left court last year after pleading guilty to causing serious injury to their toddler having fed her an extremely restrictive vegan diet

The couple's car parked outside their rental home in Queens Park, in Sydney's eastern suburbs, when they were arrested last year

'The gains have not come easily,' the carer said. 'This crime has had a long-term impact on her development.'

The girl could now crawl, stand and walk and spoke about 20 words.

She had developed a healthy appetite but her short stature meant she was obese.

The mother told police the child was a 'fussy eater'. Sometimes for a snack the child was given a mouthful of fruit or two sultanas (the parents are pictured outside court)

'It's like her body is storing calories in case she needs them in the future,' the carer said.

'She immediately stands out as different from other children,' the carer said in her statement.

The court has previously heard the mother behaved 'generally normally' when the child was first taken to hospital.

She would at times 'not engage' with her daughter but fed, changed and comforted her when staff asked her to do so.

The mother told a hospital dietitian her entire family followed a vegan diet and detailed the extreme eating measures that led to the girl's condition.

She said 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.

For dinner, she said her daughter would be offered tofu, rice or potatoes.

But she said the girl, who was also breastfed once a day, was a 'fussy eater' so she might just have oats again.

Sometimes for a snack the child was given a mouthful of fruit or two sultanas.

This diet resulted in severe deficiencies in nutrients 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'.

The couple lived with their three children in a rental home in Sydney's south until 2017 when they moved to the eastern suburbs. Neighbours said they only ever saw two children with the parents

The infant had fractures throughout her tiny body and her bones were so brittle doctors believed they could have been broken by 'normal handling'.

The girl had little documentation and no medical records after she left hospital in 2016 after being born in 'good condition'.

The mother had a water birth and discharged herself just three and a half hours after delivery. She returned for a post-natal check three days later.

However, the mother then refused a newborn screening test or a hearing check, telling doctors she would rather her GP do them.

She never saw the GP for the tests and ignored multiple phone calls, voicemails, text messages and a letter for two weeks.

'The hospital discharged (the mother) from their care after no response was received,' court documents read.

The father told told Family and Community Services the couple 'did not want to put her through immunisations as (it was) hard on baby.'

The mother (pictured with her husband) told a hospital dietitian her entire family followed a vegan diet. She said 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

The treatment of the girl was only brought to the attention of doctors in March last year when doctors attended to the infant after she suffered a seizure.

One doctor described her as 'floppy' and noted how the diminutive one-and-a-half year old didn't crawl or talk during a month in care.

Hospital staff initially respected the parents' wishes to keep the infant on a vegan diet, but grew concerned when the mother outlawed soy because it was full of 'hormones'.

Even after the girl had been in hospital for a week, the parents exchanged text messages denying their daughter's lack of growth was a result of malnutrition.

Soon after, police spoke to the father at the hospital, asking him why he wasn't concerned that his daughter never grew.

Even at more than a year old, the child weighed only 4.9kg, barely double what she weighed when she was a newborn.

'(The father) thought she was a girl and different to boys - she was petite,' court documents said.

After an investigation into the girl's medical history, doctors found an absence of immunisations, no follow-up check-ups after she was born and no birth certificate or Medicare number.

The couple's daughter was eventually put into foster care with her two older brothers, then aged four and six.

Within six months the girl had put on 13lbs (6kg) and was crawling and standing on her own.

But doctors said it was still 'imperative' she continued therapy and was constantly monitored to help manage developmental delays.

The parents pleaded guilty to failing to provide for a child and causing serious injury; charges of reckless grievous bodily harm were withdrawn.

The girl now lives with a relative and her two brothers in another state.

The sentencing hearing continues.