Why was Trans-Australia Airlines prevented from purchasing Caravelle aircraft?

Originally T.A.A. and Ansett-A.N.A. proposed to purchase Caravelle and Electra aircraft respectively. The Government -was .unable to .agree to the operators' requests .for .such radically different types of medium range aircraft as approval would have initiated a competitive race for a multiplicity of jet types. The economic results of this could have been to seriously compromise the Government's clearly stated policy of retaining two major airlines in active opposition on the main trunk routes. The -Government did recognize the need for. a medium-range aircraft 'for use -on the longer route such as the .Perth route and when the managements of the two airlines agreed to standardize upon the Electra aircraft for this purpose, approval -was accordingly given to the purchase of two of this type of aircraft by each major operator.