Explain what Curtin meant when he said, ‘But we know ...'?

The statement can throw you off a little with how it's phrased, but essentially I think what Curtin meant was, "We know that Australia can be defeated, but Britain would still hold on. We are therefore even more committed to making sure we ourselves are NOT defeated." The speech as a whole was a proclamation to create an allied tie with USA because Curtin seemed to be concerned over the fact that Britain was also having a hard enough time against Germany and that its supply line would be over extended if they were to try and aid Australia. It was a rather bold thing to say too.