HTTP Post from Android app not working as expected

Question

Trying to register a user into the database.

This code works:

try {
            URL url = new URL("http://www.jacksteel.co.uk/Comp4/registeruser.php?" + getQuery(userData));
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setReadTimeout(10000);
            conn.setConnectTimeout(15000);
            conn.setRequestMethod("POST");
            conn.setDoInput(true);
            conn.setDoOutput(true);


            OutputStream os = conn.getOutputStream();
            BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(os, "UTF-8"));
            writer.write(getQuery(userData));
            Log.d("myTag", getQuery(userData));

            writer.flush();
            writer.close();
            os.close();
            Log.d("myTag",conn.getInputStream().toString());


            conn.connect();

        } catch (Exception e) {
            e.printStackTrace();
        }

However this code does not:

 try {
            URL url = new URL("http://www.jacksteel.co.uk/Comp4/registeruser.php");
            HttpURLConnection conn = (HttpURLConnection) url.openConnection();
            conn.setReadTimeout(10000);
            conn.setConnectTimeout(15000);
            conn.setRequestMethod("POST");
            conn.setDoInput(true);
            conn.setDoOutput(true);


            OutputStream os = conn.getOutputStream();
            BufferedWriter writer = new BufferedWriter(new OutputStreamWriter(os, "UTF-8"));
            writer.write(getQuery(userData));
            Log.d("myTag", getQuery(userData));

            writer.flush();
            writer.close();
            os.close();
            Log.d("myTag",conn.getInputStream().toString());


            conn.connect();

        } catch (Exception e) {
            e.printStackTrace();
        }

The only difference being that I concatenated the result of getQuery() onto the end of the URL, which suggests that the HTTP post is not working, I got the code fore the Post from here I already have the data in a nameValuePair list called 'userData'

So, my question is why does it not work without appending getQuery(userData) to the URL, and can I make it work?