People living along the route say the roads are already being damaged by the extra trucking, and West Coast mayors raised their concerns with Transport Minister Simeon Brown during his flying visit to the region last week.
Bathurst Resources said in a statement the extended closure would result in an expected reduction in export volumes of 250,000 tonnes of coal from its Stockton mine, from a planned 1.2 million tonnes to 950,000 tonnes.
"Bathurst is updating the Stockton mine plan and is communicating with customers to align the requirement with short term production capacity."
The Tawhai tunnel, on the Stillwater-Ngakawau rail line, suffered a partial roof collapse on June 15 during track maintenance. A digger contractor noticed debris on the tunnel floor and immediately evacuated the tunnel just before the collapse.
A subsequent drone inspection inside revealed a partial roof collapse of the tunnel lining about two-thirds of the way through the 373m-long tunnel. It was constructed in the 1890s with brick lining.
Initial indications from KiwiRail were that the tunnel would be closed for two to three months.
Bathurst said the initial timeline did not affect their export plans through Lyttelton, and a road freighting plan was put in place to bypass the tunnel with a daily convoy of trucks.
However, KiwiRail had now advised Bathurst that the updated repair timeline will mean that the tunnel and rail line may not be available until December.
"The updated timeline is dependent on the assessment of the failed lining of the tunnel, the tunnel condition, remediation plan and WorkSafe endorsement to undertake the required repair works. The timeline will become clearer once the first phase of the re-entry plan is completed", the coalminer said.
The coal is now being moved by trucks through the Buller Gorge and Reefton township to Ikamatua, where it is loaded back on to trains and railed to Lyttelton.
Locals along the route said, when spoken to by the Greymouth Star that they understood the need for trucking, but they also had concerns around noise and damage to the road surface.
Ikamatua resident Roger Hampton said there were already bumps on the State Highway bridge nearby.
"They need to get in there and fix the thing [tunnel]", he said.
Another resident who lives about 2km outside of Reefton said the trucks were already damaging the road.
"There are a couple of indentations, and with trucks thumping over them, it’s getting worse."
She had noticed the increase in trucks, but said they seemed to be adhering to the 7am to 7pm curfew.
However, she did not feel there were as many as 70 trucks a day.
One truck driver said the bridge was exceptionally dangerous, as it was very narrow when a car and truck met.
West Coast mayors and chairpeople met with the Transport Minister in Hokitika and brought up the closed tunnel and general resilience concerns.
West Coast Regional Council chairman Peter Haddock said there had been under-funding of infrastructure for years.
One concern was whether Bathurst could meet its order obligations.
He said West Coast infrastructure in future needed to have many strands — from roading to rail and shipping.
