if all you've reported is correct, then _level0.container_mc.test_mc is the correct reference. if it fails, after your external swf completes loading, it's because you have more than one _level0.container_mc and/or more than one _level0.contain_mc.text_mc.
Thanks for confirming that I'm not crazy or stupid in thinking that's what the reference should be (honestly, I was starting to imagine that I must be one of the two), however I definitely only have one each of container_mc and test_mc. I've just tested this by changing the instance names (one at a time) - if there was a duplicate instance with the same name, the code would presumably still work, which it doesn't.
Thanks for the suggestion, though.
if it fails, after your external swf completes loading,
isn't true. ie, you're not waiting until loading is complete to reference that movieclip.
Thanks again for the reply, however this is also not the case. I am using the onLoadComplete() method as part of my preloading process, so I know that by the time I am referencing it, it is in fact fully loaded.
if that works, please mark this thread as answered.