The commissioning in 1972 of the Western Diversion of the Tongariro power scheme resulted in the diversion of water from eight tributaries of the upper Whanganui River. By June 1973 so significant was the diversion of waters that it was impacting on the operation of Piriaka. By the end of the year, the NZED was playing compensation to the borough council, the amount depending upon weather conditions and the amount of water they were taking. The lower river flows attracted the attention of other parties which resulted in hearings by Rangitikei – Wanganui Catchment Board in 1982 with 17 submissions and again in 1988 with 1,250 submissions being received. Unhappy with the tribunal's ruling that the NZED and its successor, the Electricity Corporation of New Zealand (ECNZ), should return river flows to full into summer and half in winter and also continue to pay compensation they appealed the decision. By September 1989 a total of NZ$510,00 had been paid to date to the borough council for loss of generation at Piriaka.
