A 10-month-old Sydney boy horrifically burnt after allegedly being left in a scalding shower is still fighting for his life more than 48 hours after arriving at hospital.

The little boy underwent surgery on Monday and remains in a critical but stable condition, a NSW Police spokeswoman said on Tuesday.

He was placed in an induced coma on Sunday afternoon having suffered serious burns to 40 per cent of his body, significant head injuries and severe bruising to his skull, face and jaw.

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His distraught mother and biological father are maintaining a bedside vigil as medical experts work to nurse their boy back to full health.

The 23-year-old man accused of leaving the boy in the shower while he allegedly smoked a cigarette and charged his phone is behind bars after being denied bail at Penrith Local Court on Monday.

He is due to reappear in court via videolink next month charged with recklessly causing grievous bodily harm.