Trucks can now cross the weathered Ashburton River bridge.
Waka Kotahi NZTA made the announcement on Thursday morning, with trucks allowed on the bridge from 10.15am.
50MAX and high productivity motor vehicles (HPMV) are among the vehicles that have been given the all-clear.
The heavier vehicles can only use the bridge between 7am to 7pm for the next week.
Currently, light vehicles have 24/7 access.
No overweight vehicles are allowed to cross the bridge.
Any trucks with an overweight permit will need to take alternate routes on local roads to get south of Ashburton, Pete Connors, system manager for NZTA, said.
A business case for a second bridge was prepared for the Ashburton District Council in September last year.
It said if the bridge was closed for any reason, using the nearest alternative road bridge involves a detour of about 60km. If the bridge was closed for a day, the total cost to the economy for additional travel time would be about $1.1 million.
The report said about 24,000 vehicles per day use the bridge.
Connors said they will keep monitoring the bridge deck and the pier for any movement during the day.
"The pier testing yesterday, using heavily laden trucks, showed negligible movement, indicating that the pier has stabilised and the structure is robust.
"We still need to take it one step at a time, however, particularly as the debris is being removed from underneath the bridge.
"After a week of use, we need to reassess the situation and decide if we can allow broader access for the freight industry."
The bridge has been closed on and off since Tuesday, after drivers reported slumping on the deck at the Ashburton end. A damaged pier had sunk around 150mm during the weekend floods.
Meanwhile, the Porters Pass/Arthur's Pass route between Canterbury and the West Coast was opened at 8pm on Wednesday night.
SH73 between Springfield and the Porter River Bridge had been closed since severe flooding damaged bridges and dumped scree over the highway.
The local state of emergency is continuing in the Selwyn District.
An emergency operations centre is open at the Selwyn District Council offices in Rolleston.
Water notices remain in place on eight water supplies:
• Arthur's Pass - Precautionary boil water/conserve water.
• Acheron - Precautionary boil water/conserve water.
• Castle Hill - Conserve water.
• Dalethorpe - Boil water.
• Hartleys - Boil water/conserve water.
• Hororata - Boil water/conserve water.
• Sheffield - Precautionary boil water.
• Springfield - Do not consume.
For residents on the Springfield supply, supplies of bottled water are available from the Springfield Challenge Station, or from the drinking water tanker at the car park of the hall/public toilets.
For residents on Hartleys supply, a drinking water tanker is at the Glentunnel Community Centre car park and supplies of bottled water are available from the Glentunnel Store.
For residents on Hororata supply, urgent supplies of bottled water are available from GAS Hororata.
- Additional reporting RNZ