Dealing with the characteristics of a usage , Halsbury says at pp. 184-186:
Every usage, whether in respect of a particular trade, branch of business or occupation, and whether affecting land or not, must be notorious, cetain, and reasonable, and it must not offend against the intention of any legislative enactment.
Every usage must have acquired such notoriety in the particular market or branch of trade or in the department of business or amongst the class of persons who are affected by it, that any person in that branch or department or class who enters into a contract of a nature affected by the usage must be taken to have done so with the intention that the usage should form part of the contract. Notoriety in this connexion does not mean that it must be known to all of the world, nor even that it should be known to the person against whom it is asserted; but it means that it must be well known at the place to which it applies, and be capable of ready ascertainment by any person who proposes to enter into a contract of which that usage would form part.
Every usage must be certain. It must be uniform as well as reasonable, and in order to be incorporated as a term in a written contract it must have just as much certainty as the written contract itself.
A usage is not , however, bad for uncertainty merely because it depends in its operation upon what a tribunal thinks to be reasonable.
66 I do not believe it was open to the Zoo to unilaterally change the terms of the employee's contract by removing the flexi day. No evidence was brought before this Commission to show that consent existed from any of the employees of the Purchasing Department for this action to take place.
67 In summary it is my finding that the evidence demonstrated a long standing usage or practice involving employees of the Purchasing Department working a particular roster that incorporated a flexi day and that this usage or practice I consider provided a fair and reasonable condition and should not be discontinued.
68 I believe it is allowable for the Zoo to formulate rosters, in accordance with the Award that in a four week( 152 hours) period, provide for both a flexi day and days of duration of hours worked that meet the needs of the Purchasing Department taking into consideration the highs and lows in the daily work load that is experienced.