Travel insurance/ health insurance for Australia

My partner and I are going to Australia on a working holiday visa at the end of the year. On our way out to Australia we are visiting south and central america, los angeles and New Zealand. We know we need travel insurance but are unsure what to get as insurance companies will only insure you for 12 to 18 months and they stipulate you need a flight home. We don't know how long we are staying and we don't have a return flight home.
Has anyone been in the same situation and what did you do? Thanks

Just get an open return ticket then the insurance.
Renew the insurance and the ticket at a later date.

Thank you, will go and speak to STA travel and see if they can help us.