I only have a few minutes so all I have time for is to address the legal standing.
Get a lawyer immediately. If the boys have been in Canada for over a year this is their legal domicile and as such you can not move legally to a different country without him giving you full custody or some other custody agreement through the courts.
If he does sign something that you can temporarily take them out of the country than you need to know that he can ask for you to come back at any time. If you don't come back, he might be able to file kidnapping charges. Plus he can hit the courts and will get full custody at that time. This means De Haag (The Hague International Agreement Canada has signed) will be on his side and if necessary will get the kids back to him through extradition from Australia. (I am not sure if Australia has rectified the agreement.)
Of course the above is the worst case scenario and would take years but it happens. If you are planing on leaving please get a lawyer first. Having a signed and notarized letter is ok for you to leave the country with but it will not be enough to stay out of the country with indefinitely.
Good luck