A Sydney couple have pleaded not guilty to neglecting their 20-month-old daughter by putting her on a strict vegan diet.

The unnamed child was admitted to Sydney Children's Hospital after it suffered a seizure last month.

Doctors called police after they discovered she was suffering from malnutrition and rickets, 7 News Sydney reports.

The couple, in their early 30s, appeared at Waverley Court on Wednesday morning where they were charged with failing to provide for a child, causing danger or serious injury and reckless grievous bodily harm.

Investigators say when the toddler was admitted to hospital her body was so malnourished she resembled a three-month-old baby.

The girl had urgent treatment and remained under medical care for three weeks.

Her parents have pleaded not guilty and are expected to appear in court again next month.

Newshub.