I needed to convert a repository from SVN to GIT a while ago and created a little script to help move everything over. It works on standard SVN repositories so if you have one that is configured in a different manor to the default it may not work as expected.
Always make a backup, or better yet run this script on a copy of the repository. Use
git svn to clone the repository, then run the script to create the tags.
git svn will create branches from the SVN tags, so this script will run through all the generated branches and convert them to tags. This should only be run on a repository that is newly converted to avoid any potential issues.
git svn clone <repo_url>