1. Do you mean
http://www.activ8ed.co.nz/administrator/ ?
2. Make sure the index.php and any other .php file has permissions 0755 or 0644 . If they are set to 0777 or 0666, it will cause 500 errors
3. Check any .htaccess files in public_html or public_html/administrator . If you see a line that starts with
RewriteRule
you must have the line
RewriteBase /
above those lines.