Prime Minister Malcolm Turnbull has confirmed that three Australian citizens and one resident were among a group of people kidnapped in a fatal ambush on their vehicle in south-east Nigeria. 
The driver of the vehicle was shot dead in the attack, on the outskirts of the city of Calabar, the capital of Cross River State, about 5.30am on Wednesday, local time, Nigerian police commissioner Jimoh Ozi-Obeh said.
Five people who worked for Australian mining and engineering giant Macmahon, which was contracted to cement company Lafarge Africa, were abducted, Nigerian police said. Peter Zoutenbier, from Brisbane, was one of those kidnapped, his family have said.
There have been reports all the workers were based in Perth and that at least one was from Western Australia.
Mr Turnbull said on Thursday he believed seven people had been kidnapped, including three Australian citizens and one Australian resident. He said the Australian high commissioner was on the scene and officials were working with local authorities "at the highest levels".
"We don't know at this stage the identity of the kidnappers. The families in Australia have been notified," he said. "In many respects the less Julie [Bishop] and I say about this the better from the point of view of recovering the kidnapped people."
Earlier, Ms Bishop said: "The Australian government is in close contact with the Nigerian government, which is taking the matter extremely seriously."
About 30 militants ambushed the vehicle near the Idundun Bridge crossing over a river, local media reported. After the attack, the kidnappers fled with the hostages aboard a boat that had been hidden under the bridge.
A witness told local media organisation Vanguard that those kidnapped were travelling in a convoy of four vehicles when the militants opened fire. "They ambushed them and killed [the driver] on the spot. One of the expatriates hid under the vehicle and they didn't see him because it was still a bit dark. They also took one of the drivers ... then one supervisor and three white guys and it was like they had a boat by the beach already waiting for them."
Two of those kidnapped managed to flee, said Irene Ugbo, a spokeswoman for Cross River State police. She did not know the nationality of the workers still being held. The kidnappers were yet to contact police, she said.
A Lafarge Africa spokeswoman said: "Macmahon is working with the security agencies to resolve this situation."