A woman who claimed Centrelink parenting payments for almost 15 years after her baby died would dress up a doll and pretend it was her daughter, a NSW judge has been told.

Alison Christie Mains, 41, is awaiting sentence in Sydney's Downing Centre District Court for three counts each of obtaining financial advantage by deception and defrauding the Commonwealth.

Alison Mains arrives at Sydney's Downing Centre courthouse on Wednesday. Credit:AAP

"The offending began as a result of Ms Mains' daughter dying in horrific, tragic circumstances, aged five months," her barrister Marty Bernhaut said on Wednesday.

He said the disabled baby girl died in mid-1998 and the debt came to the fore in 2013 - a period of 14-and-a-half years.