UPDATED 2.47PM A Perth-based mining company won't reveal if it paid a ransom to secure the release of three Australians and a New Zealander kidnapped in Nigeria.

Macmahon Holdings announced the release of seven of its workers, including the four ex-pats, on Sunday night.

Earlier reports suggested five of the seven are injured, two seriously, but Foreign Minister Julie Bishop said on Monday she believes they are all well.

The men were taken hostage and their Nigerian driver was shot dead on the outskirts of the southern city of Calabar last week.

On Monday, a spokesman for Macmahon said the company would not be commenting on whether it paid a ransom.

Ms Bishop said the Australian government had not offered any money to free the men.

"The company will have to answer for itself, but the Australian government certainly does not (pay ransoms)," she told 2GB Radio.

It's understood Macmahon is preparing to fly home the Australians and the New Zealander, who is a permanent Australian resident.

"I believe that they are all well. They are undergoing health checks. It's obviously been a very traumatic time for them," Ms Bishop said.

Nigerian police have told local media they freed the men.

"The police stormed the location where they were being held and rescued them," police spokeswoman Irene Itohan said, adding that all of the kidnappers managed to escape.

Macmahon chief executive Sy van Dyk has praised the workers for their courage.

"Our men have been through a traumatic experience," he said in a statement on Sunday.

A South African and two Nigerians were among the seven freed.

Macmahon Holdings mines material for processing at Lafarge Africa's UniCems cement plant at Mfamosing, in Nigeria's southeast.

Experts believe the Islamic militant group Boko Haram, responsible for many atrocities across Africa were not responsible for the kidnapping.

Waikato University Professor Alexander Gillespie said those responsible for the attack showed none of the violent hallmarks of Boko Haram.

"It is very unlikely that Boko Haram was involved with this. Although they like the money, they're much more involved in acts of generating media and spectacular terrorism. I think had they been involved, you would have seen a much more frontal campaign where the hostages were displayed on the Internet but here it's been dealt with behind closed doors, very professional and it's been handled very well. We should be happy."