It seems the 'parcel gold price' received by HAW is net of...

It seems the 'parcel gold price' received by HAW is net of Saracen's processing costs.

From Hawthorn's most recent annual report:

Gold sales are recognised based on the quantity and grade of the ore mined and processed at Saracen Mineral Holdings Ltd’s (“Saracen”) Carosue Dam Mill processing operation. The amount paid by Saracen is the amount billed, net of processing costs borne by Saracen.  (page 20)

(i) Trade receivables relate to ore sales to Saracen Mining. All amounts are billed and due in accordance with the Ore Sale and Purchase Agreement. Under the agreement all amounts disputed due to ore quality are settled through an independent umpire. The Agreement deals with the processing of ore by Saracen and includes a discount on processing costs that is repayable to Saracen. As a result, the trade receivables balance represents a ‘rolling balance’ made up of initial sales invoices, payments, adjusting sales invoices, processing cost offsets and discounts. Consequently, there is no aged debtors listing. At 30 June 2018 the balance of trade receivables is not considered impaired.
(ii) Accrued revenue relates to ore supplied to Saracen Mining but not yet invoiced. The final amount invoiced is subject to adjustment based on ore quality. (page 30)

The bottom line is that the arrangement is beneficial for the shareholders of both companies.