The two contestants are Chef Gino D'Acampo and Hollyoaks actor Stuart Manning. The show was aired last year.
RSPCA's chief inspector, David Oshannessy said a fine of A$3000 (£1,903) has been imposed on ITV. He added that the fine reflects the fact that all animals are protected by the Prevention of Cruelty to Animals Act.
Responding to this, ITV said it will make sure something similar does not happen again in the series.
A company spokesman clarified: "The production was unaware that killing a rat could be an offence, criminal or otherwise, in New South Wales."
He added that the incident was an oversight and ITV is putting in place a "comprehensive training programme" to make sure it is not repeated.