Two West Coast councils say the increasing costs of maintaining the only roads into Karamea and Jackson Bay could lead to extra rate rises of 5% in Buller and 8.2% in Westland.
The Karamea Highway and Haast-Jackson Bay Rd are now funded as special purpose roads.
In 2013, the NZ Transport Agency reviewed its funding policy and decided to remove the special purpose road category, with a transition plan through to July 2024.
Westland and Buller councils have jointly written to NZTA expressing concerns at the spiralling costs to look after the roads.
"Our primary concern is that removing the special purpose road category ... could render the maintenance costs unaffordable for our ... councils," they said.
The rate increase needed to keep the roads open was
"not affordable to the communities."
In the past decade the risk to the roads had shifted due to the increased frequency of adverse weather events, which put the long-term sustainability of funding the special roads at risk.
Buller and Westland had a small population, and some of the highest deprivation rates in New Zealand.
The 62km Karamea road accounts for 11% of the Buller council's total road network, and the 49km Jackson Bay road is 7% of Westland's network.
Coastal erosion, bridge painting and structural reviews on the Haast-Jackson Bay Rd meant a further $6 million-$7 million of expenditure was required within two to five years.
Overall costs are forecast to increase for the Haast to Jackson Bay Rd from $2.8m for 2018-21 to $7.8m in 2024-27.
"As of now, Buller District Council and Westland District Council are unable to approve any transition plan, as the intricacies of the situation demand ... collaborative efforts to arrive at an equitable and viable resolution."
