The "slumping" Ashburton bridge has reopened for light traffic, says Waka Kotahi, the New Zealand Transport Agency.
It had been testing the damaged bridge on Tuesday afternoon and evening, and at 11.35pm traffic started moving again over the flood-affected State Highway 1 link.
The bridge will have two-way traffic, restricted to 30 km/hour in each direction.
However, it is expected to close again for about four hours today for more tests, between 10am and 2pm.
Ashburton mayor Neil Brown earlier told RNZ 14 tons of concrete blocks would be put on the section of the bridge that had slumped - a stress test to see whether it could take the weight.
If it did not move, light vehicles should be able to allowed through, he said.
Pete Connors, System Manager for Waka Kotahi, said in a statement the damaged pier was stable.
"Based on these results under load testing pressure each lane, the bridge can take light traffic," he said.
"Waka Kotahi's bridge engineers are working on the longer-term solutions for this bridge's pier and we will keep the community informed as that is developed," Connors said.
Brown told Checkpoint the stress-testing process would take up to four hours.
Alternative routes were being looked at for trucks and other heavy vehicles via local roads, adding about 30 minutes to their journey, he said.
"There'll definitely be a bigger fix coming."
They had been looking at a bypass road around Ashburton and upcountry for heavy vehicles but had not got it open like they thought they would have, Brown said.
There was a route open for emergency vehicles, so Ashburton was not completely cut off for emergencies from the south, he said.
The testing would be carried out by a crane, which would drop 12 concrete blocks totalling 14 tonnes on the damaged section of the bridge.
The bridge is a part of State Highway 1, and having it out of action means access to either side of the Hakatere river cut off.
It was closed immediately after motorists reported they could feel it slumping underneath them.
Typically, there would be 21,000 vehicles going over the bridge on a weekday, peaking at 30,000 vehicles per day on weekends, Brown said.
Flooding today had cut off other routes south of Ashburton, which meant no alternative route south.
Brown said earlier today one section of the bridge had slumped four or five inches and there were cracks that would fit a hand in.
"It's an old bridge but it's not due for replacement. But what we have been planning in the last 10 or so years is an alternative bridge, about 500 metres from that bridge, and we're due to build a new one in 2025 and we've been planning for that for 10-15 years.
"We might be a few years late in that planning, going by today's event.
"We have had big floods in the river before, but nowhere near as big as this flood. This is the biggest flood in that river there has ever been."
Meanwhile, gaps are appearing on lower South Island supermarket shelves after floodwater damage cut key delivery routes through Canterbury.
Countdown and Foodstuffs supermarket chains confirmed the road closures would impact lower South Island deliveries and they were working to find alternative routes, but that could mean significant diversions and delays for delivery trucks.
MetService said no more rain was forecast for the Canterbury region for the rest of the week.