Three Australians, a New Zealander, a South African and two Nigerians were freed on Sunday, four days after they were kidnapped in an ambush on a police convoy in southeastern Nigeria that killed a driver, officials said.

No ransom was paid, according to Assistant Superintendent Irene Ugho, spokeswoman for the Cross River state police. She described the release as a "rescue exercise" involving security agencies but provided no further detail.

State security adviser Jude Ngaji, the seven mine workers' employer Macmahon Holdings and the Australian government confirmed that all seven had been freed.

Western Australia-based Macmahon said in a statement late Sunday that five of the men were injured, two seriously. They were receiving attention from a medical team. Macmahon did not identify the five.

But Australian Foreign Minister Julie Bishop told 2GB Radio in Sydney early Monday: "I believe they are all well, undergoing health checks, and it's obviously been a very traumatic time for them and they are all returning home to their families."

She said the Australian government did not pay any ransom and would not say if Macmahon had.

Macmahon chief executive Sy van Dyk praised the men for the courage they displayed throughout the ordeal.

"Our men have been through a traumatic experience and we have mobilized medical and other support teams in Nigeria to provide immediate support," he said.

"I also thank the men's families for working so closely with the company during what has been an extremely difficult time for them as well. They too have endured an incredibly stressful experience," he added.

The families had been told of the release and had spoken to their loved ones, he said

Police had identified the kidnapped foreigners as Australians Jack Countentz, Mark Gabberdy and Peter Zoutenbier; New Zealander Jamal Khan, and South African Wayne Smith.

They were snatched at gunpoint along with two Nigerians on Wednesday and forced from their vehicles, which were in a convoy escorted by police near Calabar, the state capital.

It is not known if the perpetrators of the latest kidnapping made a ransom demand.

Kidnapping for ransom is common in Nigeria but generally involves no fatalities. Hostages are returned unharmed once money exchanges hands, though a German construction worker was killed in southwestern Nigeria late last year by gunmen who kidnapped a second German. He was later released.