Infrastructure and mining services group Downer EDI has agreed to buy New Zealand builder Hawkins for an undisclosed sum, as it expands operations in the Pacific nation.

The Australian company called the acquisition an excellent strategic fit, saying it will complement existing capabilities and provide a platform for growth.

Hawkins is one of the largest construction firms in New Zealand, with current major contracts including building Wellington Airport's new control tower, Christchurch's town hall and Auckland's Park Hyatt hotel.

The company was founded 70 years ago, and is being purchased from the current owners - the McConnell family.

Downer chief executive Grant Fenn said the acquisition would help his company benefit from the booming construction activity in New Zealand, where an estimated $NZ50 billion ($A45.9 billion) is expected to be invested in non-residential construction, over the next five years.

Hawkins will continue to operate under its own brand.

The acquisition, expected to be completed by March 31, will be funded through existing debt facilities and be earnings accretive in its first year, Mr Fenn added.

Downer in February posted higher half-year profit and raised full-year guidance, suggesting its shift to rail, transport services and technology sectors was helping to offset the continuing decline in its core mining services business.

Downer has built capability in other segments, particularly in light rail design and construction, and utilities services, as it continues to reposition the business to service increased investment and outsourcing in public transport, communications and defence sectors.

At 1055 AEDT, Downer EDI shares were unchanged at $7.21 each.