I have a sleep apnoea machine and I am going to China.

If you have the manual for your machine you should be able to check the acceptable range of input voltage. As long as it is 220-240V or more, yes. Most appliances nowadays are capable of accepting voltages from 100 to 240V. You may need to buy a plug adapter. From what I understand plug standards in Oz and China are the same so you should be ok but if you can't plug it in, it should be easy to get a plug adapter locally. In other words, you'll probably be fine and just need a plug adapter at most.