On 28 December 1960 Dr. Weeks, an American petroleum geologist, entered into a consultancy agreement with the third defendant, The Broken Hill Proprietary Company Ltd. ("B.H.P."), to advise it in its search for crude oil and gas. Under the consultancy agreement B.H.P. agreed, amongst other things, to pay to Dr. Weeks or his nominee an overriding royalty of 2½ per cent of the gross value of all hydrocarbons produced and recovered by B.H.P. within, amongst other areas, the area in Bass Strait covered by an exploration permit taken out on the advice of Dr. Weeks. B.H.P. granted to Oil Basins Incorporated ("O.B.I.") the overriding royalty for which the consultancy agreement provided. By a series of assignments, the obligations under the royalty agreement were assigned to the fourth defendant, B.H.P. Petroleum (Bass Strait) Pty. Ltd., and the right to receive the royalty was assigned to the plaintiff, Oil Basins Ltd. On 12 May 1964 the fifth defendant, Esso Australia Resources Ltd., then known as Esso Exploration and Production Australia Inc., farmed into the relevant exploration permit area subject to a 2½ per cent overriding royalty in favour of O.B.I.