A Diploma program is equivalent to the first year of an undergraduate degree and is typically offered in Australia. The entry requirements are lower than those of an undergraduate program but are designed to assist students to be well prepared to join their peers in year 2 of the undergraduate program. The duration is about 6-14 months and is in the same field of study as the bachelor program you intend to graduate from.
Almost all Diploma programs are affiliated with a specific university and you will be guaranteed into Year 2 of an undergraduate course upon successful completion. Eventually, you will graduate with the same degree as your peers in the same duration of time.
A-Level/IB/Diploma/O-Level -> Diploma (1 year) -> Undergraduate (2 or 3 years)