Before it can become a law, a bill must be agreed to by both the Senate and the House of Representatives in precisely the same form. If there is a disagreement over amendments or requests, a bill may be transmitted between the two Houses a number of times until the Houses finally agree to it in the same form or it is abandoned. These negotiations on amendments and requests are conducted by the consideration of messages between the Houses.
Brief Guide to Senate Procedure No. 16 - Consideration of Legislation
Brief Guide to Senate Procedure No. 18- Communication between the Houses - dealing with messages
Senate Brief No. 8- The Senate and Legislation