1. I think the 30 days is stay duration of each entry. You can enter China for countless times within the 6 months and stay in China for 30 days each time.
2. If you stay in China for 30 days each time, you must exit China compulsory.
3. yes, you can re-enter China after permissible period with this visa withour applying for a new one.
4. yes, you can re-enter China even though you exit China and turn your head to enter again.

If you apply in Australia you should get a 6 month multiple entry L visa, 60 days per entry. I recently obtained a 12 month one. The 30 day limitation is often imposed when you apply within China (mainland. Various restrictions may be imposed when youn apply in another country.

If you have a multiple entry visa you can re-enter as many tiomes as you like, each for the duration you have been granted, 30 days, 60 days or whatever. there is no minimum time that you must be out of China, I have just walked back in as soon as I was admitted into Macau.

Basically, you can only apply for one new visa (often erroneously called an extension) within China (qand as we just said that is normally for a max of 1 month. However if you seek a second one you will usually be given 7 days only.

Agents thrive on telling you how hard it is to get a visa. On the other hand the genuine ones do have contacts and can get better visas than you might get yourself. The fee in Australia varies around $100 depending on duration. The fee on the mainland should be around 200 rmb.

Correcvtion to my previous reply.....

Sorry I mixed up this query with another that specvified Australian. Please ignore my iniitial comments... the rest are valid.