On QantasLink services, from the rulebook......
Flight deck visits by commercial passengers (regardless of age) are not permitted. Flight deck access is restricted to aircrew on duty, approved persons on staff travel, employees of clients, and regulatory officials who require access for operational purposes.
Approved person and employees of clients means someone approved by company head office prior to flight, and whose bona fides have been checked prior to that approval. You would need an ASIC card or equivalent employer issued photo ID, plus a plausible reason for being up the front.
It's a shame, but there it is. Better plan on sitting in the cabin.
p.s. PIC still has the final say, within those limits.