Many of the single trip products that appear here on Money Supermarket will probably give you cover whilst you are travelling to your final destination with some continuing for a short period after you arrive.
You haven't mentioned whether you are stopping off anywhere along the way but if you do a normal search on the MSM calculator to cover the duration of the trip to Australia you should get quite a few options. Review the policy wording, within the definitions on the policy most will mention one way trips and explain what that companies rules are. If you are in any doubt after reading the wording then call the provider for confirmation.
Hope this helps.