What is a 2:1 Honours degree and the Australian equivalent?

A 2:1 Honours degree is shorthand for Upper Second Class Honours. It refers to the level of achievement in an undergraduate (Bachelor's) degree (usually requiring 60-69% for the cumulative grade).
https://en.wikipedia.org/wiki/British_undergraduate_degree_classification

In Australia, there is no exact equivalent, as most universities do not differentiate level of achievement in the bachelor's degree, but require a further year of study to obtain an Honours degree. The standard of work during that year determines the level of honours (1st, 2nd, 3rd or none) which is awarded.
https://en.wikipedia.org/wiki/Bachelor_of_Arts