The crash follows a horror night on New Zealand roads, after three people died and another was critically injured in a two-vehicle crash in the Coromandel last night.
The bridge south of Thames is closed in both directions following the accident.
Police said there were two people in each vehicle.
Emergency services were called to the south side of the bridge at 8.30pm last night.
A total of 125 people have died on the country's roads so far this year - 18 more than at this point last year.
These deaths have occurred in 109 fatal crashes - 12 more than last year.
Police were called at 6.30am yesterday morning to another two-car crash on SH2 just north of Dannevirke, where a motorist died.
