Accepting the regulation as a valid legislative provision the initial question which arises is concerned with the character of the function which by virtue of the language of the condition specified in the Third Schedule must be taken to have been entrusted to the Director-General. In particular, the question is, does the language in which the condition is expressed create a duty the performance of which may be enforced by mandamus? No right, conditional or otherwise, to the Director-General's written permission is expressly given by the regulations and the question of the existence of a duty, and a corresponding legal right, capable of enforcement by mandamus, is left to be resolved as a matter of implication. Whether or not any implication favourable to the prosecutor should be made must, of course, be considered in the light of the fact that reg. 4 (2) and the Third Schedule is found in subordinate legislation which places no limits upon the considerations which are to guide the Director-General in the performance of the function entrusted to him and which is of a type which gives no clue to any intended limitations. It is legislation promulgated under the Customs Act and is not concerned with any particular subject-matter capable of revealing the scope and object of the function. It may, of course, be that some clue as to the appropriate considerations may be found in the fact that the Director-General is the permanent head of a department of the Commonwealth established for the purpose of exercising a large measure of control in relation to civil aviation of all kinds within the Commonwealth - whether it be inter-State, intra-State, or overseas - and that the function is committed to him in that capacity. But whatever clue this circumstance may afford is vague and indeterminate and can indicate only that he is invested with a discretion which is to be exercised in accordance with his own views as to what will best serve the interests of civil aviation within the Commonwealth. It should also be borne in mind that his function is not a function to prohibit; his authority extends only to the relaxation of a prohibition erected by the Regulations and which, itself, is not in question. The situation is, we think, not unlike that which arose for consideration in Metropolitan Meat Industry Board v. Finlayson [1] where this Court was concerned to enquire whether mandamus would lie to compel the performance of a duty said to arise by virtue of the provisions of s. 19 of the Meat Industry Act, 1915 N.S.W.. That section was in the following terms: "After this Act comes into force - (1) No person shall, except with the consent of and under the conditions prescribed by the" Metropolitan Meat Industry "Board, within the metropolitan abattoir area, slaughter any cattle or dress any carcase for human consumption, except at a public abattoir". Griffith C.J. (with whom Barton J. agreed) was of the opinion that this section did not create any right enforceable by mandamus where consent had been refused for undisclosed reasons. He said: "I asked in vain during the argument for some indication of the matters to which the Board were required to direct their minds in considering such an application, but received no answer except that the Act contemplated that private slaughtering establishments should continue to be carried on as before, and that consent should therefore always be given unless some special reason should be shown for refusing it. It was therefore, it was said, the duty of the Board to indicate to the applicant the reasons which they thought to exist. The answer to this argument is that the Act expressly negatives it. It says that what has hitherto been conditionally lawful shall in future be unlawful except on one condition. In my opinion the discretion of the Board is unfettered. If they think, for instance, that it is in the public interest to confine all slaughtering to the Government abattoirs I do not think that any Court can review their opinion" [1] . Isaacs and Rich JJ. were of the same opinion and observed that: "The only duty the Board has is to receive and consider any application for consent, to honestly deal with it, and to give or refuse its consent as it thinks the public interest in the performance of its mandate in s. 13 requires. The main and overriding consideration is that mandate of s. 13; if consistently with its own view of discharging that great function it thinks, for whatever reasons commend themselves to the Board, that the public welfare will be better served by granting or by refusing its consent, it should act accordingly. No Court can prescribe any narrower or stricter limits of duty" [2] . In our view, much the same observations may be made concerning reg. 4 (2) and the language of the relevant condition set out in the Third Schedule. Whether permission is to be given or not is a matter left to the discretion of the Director-General. His discretion, no doubt, is intended to be exercised upon broad considerations relating to civil aviation in the Commonwealth but this circumstance does not support any implication that there is reposed in him any public duty, or any legal right created - other than a right to have an application for permission honestly considered - which is capable of enforcement by mandamus.