Three Australian workers kidnapped from a worksite in Nigeria have been released, police and engineering giant Macmahon Holdings say.

Key points: Men among seven released by kidnappers, including a South African, a New Zealander and two Nigerians

Men among seven released by kidnappers, including a South African, a New Zealander and two Nigerians Two of the seven seriously injured

Two of the seven seriously injured No word on whether a ransom was paid

No word on whether a ransom was paid Macmahon thanks local authorities for help in securing release

The men, who work for the Perth-based mining company, were attacked by gunmen on Wednesday.

Their Nigerian driver was killed in the assault, and a fourth Australian man escaped.

Commissioner of police in Cross River state Jimoh Ozi-Obeh said seven men were released by the kidnappers, including the three Australians, a New Zealander, a South African, and two Nigerians.

No ransom was paid and no further details were disclosed, Mr Ozi-Obeh said.

Macmahon chief Sy van Dyk said the men had had a chance to speak with their loved ones.

"Our men have been through a traumatic experience, and we have mobilised 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."

Mr van Dyk said five of the men were injured, two of them seriously.

Cross River state security advisor Jude Ngaji said only the two Nigerian workers had injuries, while one or two others had "some scars".

Macmahon praises local, foreign authorities

The workers were attacked by about 30 militants near Calabar, about 600 kilometres south of the capital Abuja.

The four-car convoy was set upon early on Wednesday morning, and an eyewitness reported seeing the victims bundled into a boat.

Macmahon Holdings in Nigeria Operates two major projects in Nigeria

Operates two major projects in Nigeria US$105m cement plant at Ewekoro — 6.5 year contract with French firm Lafarge

US$105m cement plant at Ewekoro — 6.5 year contract with French firm Lafarge $US18m per year joint venture with Lafarge, Holcim and Flour Mills of Nigeria at Calabar

$US18m per year joint venture with Lafarge, Holcim and Flour Mills of Nigeria at Calabar Recent industrial action at Ewekoro resolved. Calabar not affected

Recent industrial action at Ewekoro resolved. Calabar not affected Financial half-year ending December company notes Nigeria operations failed to perform to expectations

Financial half-year ending December company notes Nigeria operations failed to perform to expectations Corrective actions to be undertaken at Calabar

Corrective actions to be undertaken at Calabar Ewekoro operations to conclude June 2016

Macmahon has a seven-year contract for quarrying operations in the region.

The Niger Delta is a rich oil-producing region and there are a number of criminal networks that operate in the area.

It is not uncommon for foreign workers to be taken and later released after a ransom payment.

Unofficially, some European countries will pay ransoms, but it is the policy of Australia and New Zealand not to pay to secure the release of citizens.

Mr van Dyk said both local and foreign authorities had been crucial in securing the men's release.

"We are very grateful for the professional support we have received from the authorities on the ground in Nigeria," he said.

"The assistance of local and federal agencies has been invaluable.

"This has been an incredible team effort and our highest priority now is to finish the job by continuing to work together to get our people back safely to their families and homes."