Will I be able to enter South Korea as a tourist with a one-way ticket and no ticket home yet?

According to TIMATIC (the Visa processing system use by most airlines/travel agents since 1963) the requirements for an Australia citizen visiting South Korea are :

Passport required.

• Passport and/or passport replacing documents must be valid on arrival.

Visa required, except for Those traveling to attend conferences, exhibitions, meetings or for touristic purposes:

• for a max. stay of 3 months for nationals of Australia;

Additional Information:

• Visitors are required to hold proof of sufficient funds to cover their stay and documents required for their next destination

There is no description as to what "documents required for their next destination" means. This is NOT the wording generally used to state that you require an onward ticket - "documents" normally refer to Visas/etc, but asking you to show a visa for your next destination doesn't make much sense if you don't have a ticket there! In your case if your next destination is Australia then technically no additional "documents" would be required beyond your Australian passport. However without an onward ticket that may or may not be sufficient.

As with all Visa questions, the final decision is up to the immigration officer at the border. This may be a situation where emailing the local South Koreans embassy/consulate and asking this question would be a good idea (I would email rather than call so that their response is documented).

A couple of things, based on experience.

1. I have more than 100 entries in Korea, and I never was asked once by Immigration for a plane ticket leaving Korea. Even back in the days or red carbon copies tickets.
2. Every single time I flew to Korea from Japan I was asked for an onward ticket, or a residence card by the airlines. Never fails. As a long-haired backpacker, as a bald businessman, every single time. Never happened from any other country. So depending on the airline, as was mentioned in the comments, it's more the airline than immigration that's an issue.
3. Good luck trying to get a Chinese visa in a country where you're not a resident. Don't even bother trying in Seoul, it won't work. Korea is probably the worst place to try. Your only serious option would be to do it in HK, where they're more lenient (and I'd recommend doing it through a big travel agency, like CTS or Travel Expert).

You will need some form of onward ticket. Korean immigration doesn't even enter into it: Without the ticket the airline will not even let you board the flight to Korea.

The Airline will check the TIMATIC system mentioned in the other answer, and make sure you have all the necessary documentation. The reason is that if the don't and you fail to get through immigration, it becomes the airline's problem, and they really want to avoid that.

You can of course buy some refundable (or very cheap) onward ticket and not use it.

Personal experience: When we went to Korea on a one-way ticket (via HK on Thai) they would not let us check in online. They checked the onward ticket at the counter before we could get our boarding passes. Going from Korea to Japan it was the same thing.

You can DEFINITELY get a Chinese visa at the Chinese Embassy in Seoul. You must simply prove that you have legal status in Korea and are not within 6 months of the expiration of said status. If you ARE within the six month window but have an old, expired Chinese visa, show it and they should waive the six-month rule. If all else fails, it’s quite possible (for my fellow Americans anyway) to fedex your passport and visa documents to a visa courier company in the US and they can apply for it for you there.

What I've done in the past when immigration might require a departing ticket but I didn't have a pre-determined departure date: Book a separate departing ticket but don't pay for it. Print the reservation with your information (name and whatever else) showing. Show that if asked, then if pressed, tell them your plans have changed and you haven't booked your new ticket yet.

I don't recall every actually having to show the departing "ticket".