The Waitaki District Council says it fears a historic bridge, which could cost up to $10 million to replace, has been seriously damaged by floodwaters.
The council and Civil Defence Waitaki met on Tuesday morning to assess the damage to North Otago from a two-day deluge, believed to be the worst since flooding in 2010.
Council chief executive Michael Ross said he estimates that road repairs alone will cost the district about $600,000.
However, Mr Ross said he is worried that the timber Kakanui Bridge, which was hit by floating tree trunks on the hugely swollen river on Monday, could be weakened and would cost $8 million to $10 million to replace. Engineers are assessing it.
Flooding in Otago is receding but more than 40 roads remain closed on Tuesday, after rivers rose to record levels, cutting off towns and farms. At least four families are isolated near Kurow and in Danseys Pass where a bridge was washed away.
The worst flooding was of the Kakanui River which had more than 100 times its normal flow.
The New ZealandTransport Agency said some roads are still closed, including State Highway 90 at Tapanui in South Otago. Part of State Highway 1, north of Oamaru is still closed, but a detour is in place.
Waitaki District mayor Alex Familton told Radio New Zealand's Morning Report programme on Tuesday he had been out to check river levels and the water was down. He said it was too early to gauge how much damage had been done, but he suspected many of the badly damaged roads will be costly to fix.