The families of Mark Gabbedy and Jack Couranz, two of three Australian workers kidnapped from a worksite in Nigeria, have expressed their relief the men have been released after the "unbelievably stressful" experience.

The men, who work for Perth-based mining company Macmahon Holdings, were among seven who were released last night after being kidnapped by gunmen on Wednesday.

Five of the seven men were injured during the attack, two of them seriously.

A New Zealander, a South African and two Nigerians were also released.

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

In a statement, Mr Gabbedy's elderly father Colin described the experience as "unbelievably stressful".

Colin, who has spoken to his son by phone since his release, said Macmahon Holdings representatives had been in constant contact with the family.

He said the company had demonstrated a "fierce determination" to secure the release of the hostages.

Machinery and workers at Macmahon Holdings' Calabar site in Nigeria. ( Supplied: Macmahon Holdings )

"We are enormously grateful to them," he said.

The Couranz family issued a statement echoing the feelings.

"Obviously we are very relieved — we have spoken to Jack and he seems in very good spirits. We are looking forward to having him home," they said.

"We are extremely happy with the way Macmahon handled the situation and are grateful for the results that were achieved."

Crews worked 'around the clock' for men's release

Macmahon Holdings CEO Sybrandt van Dyk told reporters on Monday all seven men were now in a "safe location", but five had been injured during their ordeal.

"Three of the men have wounds and two remain in a serious but stable condition," he said.

"Two of the men have rib injuries. They all are receiving specialist medical attention.

"Our priority now is to ensure that all of them are stabilised and ultimately given [clearance]... to travel."

He said the men had all spoken to their families overnight, which had been "a great relief to everyone".

"Their families have been through a very stressful and anxious time," he said.

"We will continue to provide them with all the support they need.

"I would like to offer my condolence to the family of our local driver who was killed during the incident. Our thoughts and prayers are with his family."

Mr van Dyk thanked those who had contributed to the group's release, highlighting the work of Nigerian authorities.

"Last night's outcome has been the result of a great team effort - in particular I would like to thank the Nigerian authorities, at both the local and federal level, who have provided us with professional support every step of the way, and assisted us with the safe recovery of our men," he said.

"I'm also grateful for the efforts of the Australian, New Zealand and South African authorities and to a team of specialist international security advisers who have worked with us to help secure this outcome."

Foreign Minister Julie Bishop earlier also thanked Nigerian authorities.

"We've worked very closely with the families of those who have been released, and with Macmahon Holdings," Ms Bishop said.

"And I want to place on the record our thanks to the Nigerian authorities at a state and national level, who have cooperated with the Australian Government and Macmahon Holdings to secure their release."