The current ClickOnce based installer does not allow offline installation of SourceTree. This is a problem for us because we use SourceTree on machines on a island network, i.e. one that is not connected to the Internet.
Our web proxies also block direct download of executables for security reasons and so prevent us from downloading the ClickOnce bootstrap.

Please consider moving to an MSI based installer (I can help you out with this if you wish! ).