Australian citizens visiting the UK as tourists are eligible to [use the eGates](https://www.gov.uk/government/publications/coming-to-the-uk/faster-travel-through-the-uk-border), so your daughter may breeze through Immigration with no questions asked. However, if for any reason there’s a problem (her passport is rejected, random selection, an out-of-order eGate, big queues to use them, etc etc), then she will have a face-to-face landing interview, which may or may not prove problematic. 

From [Gov.uk](https://www.gov.uk/check-uk-visa/y/australia/tourism)
>You must provide a valid passport or travel document. Your passport should be valid for the whole of your stay in the UK.
>
>You may also be asked to prove that:
>
>- you’re visiting for tourism
>- you’re able to support yourself and your dependents during your trip (or have funding from someone else to support you)
>- you’ve arranged accommodation for your stay
>- you’re able to pay for your return or onward journey (or have funding from someone else)
>- you’ll leave the UK at the end of your visit

The standard advice for non-visa nationals to avoid a potentially unpleasant situation (including the possibility of being denied entry), is to carry some evidence that proves all of the above *in case it’s needed*. The easiest evidence proving intention to leave is a return ticket, but it could include, for example, a credit / bank card with sufficient available funds to buy one.