Humans are fallible. Nowhere is this more so than in a government department. Bearing this in mind, you need to think very carefully before making the significant financial commitment that is required when entering national phase.
When filing and prosecuting an international application in Australia, you should bear in mind that the application is examined by an Australian examiner. The examiner represents the International Search Authority but is not actually a member of WIPO sitting in Geneva or The Hague. As such, the search is carried out to Australian standards. It is a terrible thing to generalise. And I am sure that most Australian examiners are competent. But my understanding is that the examiners are directed to work under certain constraints. For example, an examiner cannot simply follow his or her nose and keep digging up relevant prior art. The search fee component of the international filing fees is a healthy $2200, so you would think that the search results would be adequate. However, more often than not either a European, Japanese or US examiner finds prior art not uncovered by the Australian examiner. This can be devastating.
Let's consider national phase in 4 or 5 countries. That's between $20K and $30K, depending on various factors. What would you pay for some assurance that the first report issuing out of one of those countries does not blow your entire portfolio out of the water? Even 20 percent is $4000 minimum. You could have a very useful search carried out for that amount. It would not be unreasonable to consider more, particularly for more national phase countries.
You should note that an adverse examination report in one country can be just as relevant in all the other countries. It happens regularly that a Japanese report, which is usually delayed, ends up making a big hole in a portfolio. In other words, you could spend literally tens of thousands of dollars before finding out that it is not, after all, possible to achieve patent protection in most countries.
So, searching must remain on top of your list of priorities. You may have searched before filing the provisional patent application. But you should still not assume that all is well. Searches are usually carried out to a budget. The more you spend, the more thorough the search.