Almost the same thing happened to me when I traveled to the UK last time.

You better have proof of everything: Your return ticket, hotels/accommodations for every day, and sufficient cash for every day. It is better if you speak good English and have at least a vague idea of what you plan to do in the UK.

We don't have the full story. If this awek only bring AUD 350 for 5 days, don't have any accommodations booked, can't show return ticket and only going to Australia 100% relying on CC, then it might be reasonable to reject.