They won't let you in unless you have an outbound ticket.
Maximum length of stay in Indonesia is 60 days. Requests for Tourist Visas are submitted to the Indonesian Embassy in Canberra by filling out the appropriate forms (1 copy for each person) and enclosing:
6. Proof of a ticket in and out Indonesia, or a ticket to continue travel to another country.
www.kbri-canberra.org.au/visa/type_tour.html