A toddler who lay on top of her murdered mum in a sewer for 80 hours in Argentina has been rescued alive.

The girl's father, Gonzalo Lizarralde, allegedly killed his estranged ex-wife, Paola Acosta, and then dumped her and 21-month-old Martina down a waste pipe in the city of Cordoba, about 700km northwest of the capital Buenos Aires, reports local radio station Cadena .

He left the girl to die reportedly because he could not bring himself to kill his own daughter as well.

Martina was spotted by a local baker worker who lifted up a manhole cover to find her surrounded by filthy water.

She was transported to hospital where she is being treated for a leg fracture and pneumonia.

Police had been searching for Martina and her 36-year-old mother since last Wednesday when Ms Paola's two sons from a previous relationship reported them missing.

The pair had disappeared after Lizarralde had gone to pay his ex-wife child support. It was the first time he had seen his daughter after being forced to recognise her as his following a DNA test.