The bridges closed at 8pm and were due to reopen today after 7am when the scouring had been reassessed, New Zealand Transport Agency's highways operation manager Peter Connors said.
The scouring did not appear to be major. The closure was a precaution until a more detailed inspection could be made today.
"The decision to close the bridges overnight has been made because it would be too difficult to detect any further scouring when it is dark. River flows at the bridges are very low at the moment," Mr Connors said.
A decision on further closures of the bridges would be made after the pier and its piles had been fully assessed.
Emergency services had been notified, and had made alternative access arrangements in the event of an emergency.
This will be the fifth time the bridges have been closed this year, including for nine days in January when flood flows damaged piles. Since then closures have been to remove willows from around piles and for maintenance, inspections and repairs.
Closures earlier this year led to complaints about the bridges, public meetings, a petition and the threat of "No Bridge, No Vote" in this year's general election; along with fears another flood could bring the bridges down.
At the end of September, Waitaki MP Jacqui Dean announced funding would be made available to replace the 131-year-old single-lane bridges with new two-lane concrete structures. Work would begin next year, due for completion in 2014 or 2015.
Early estimates had put the cost of the work at about $15 million.
