A joined union would be more efficient and better resourced than the separate organisations, said Service and Food Workers' Union national secretary John Ryall.

The old adage of strength in numbers is the main driver behind the proposed merger between two of New Zealand's larger unions.

In July, the potential combination of the Service and Food Workers Union (SFWU) with the Engineering, Printing and Manufacturing Union (EPMU) will be put to the vote at a national conference and if it gets the thumbs up there, the memberships of both groups will then get to have their say.

The merger could result in a more than 50,000 strong union being established, which would make it the second biggest group behind the Council of Trade Unions. The current membership of the SFWU is about 21,000, while the EPMU numbers sit around 32,000.

Industries covered by the union include people working in the food, manufacturing, communications and engineering sectors.

EPMU national secretary Bill Newson said discussions have been underway for about three years between the two and he described the process as "complex."

"No one can say we rushed into it," he said.

He said mergers were not uncommon but the scale of what was proposed between the SFWU and EPMU set it apart from others completed in recent times, including the February merger between the EPMU and Flight Attendant and Related Services Association,

"What's rare about this one is the scale of it," Newson said.

Newson said while the idea to join together made good economic sense and provided the groups with an ability to share resources, it will also support the campaigns the two unions already worked on together.

"The driver is essentially building union strength," he said.

While membership numbers were not expected to sky-rocket if the two groups did become one, he said he hoped those represented by the union would get "more consistent and high value representation," he said.

Newson said he had "pretty positive" feedback about the proposal so far but it will be ultimately up to the membership to decide on whether to accept it or not.

John Ryall, SFWU's national secretary, said the move made sense from his perspective.

"We already have many work sites that have both SFWU and EPMU members. There are economies of scale which mean the new union will be more efficient and better resourced than the SFWU and EPMU on their own," he said.

Ryall said efforts to address issues like low wages, insecure hours and poor health and safety standards would be bolstered by an ability for to the unions to come together and "win a better deal for New Zealand workers and their families".

If it gets the green light, a refreshed governance structure will be established and a new name and logo will also be developed in time for its launch in October.