E tu, the largest private sector union in New Zealand, is donating $120,000 to Labour and $30,000 to the Greens for the general election in September.

The union said the amounts decided by the national executive are similar to those given by its legacy unions in 2014 when the last general election was held.

In 2015 the Engineering, Printing and Manufacturing Union (EPMU) and the Service and Food Workers Union merged to form E tu, which represents more than 50,000 workers.

Only the Public Service Association had a larger membership, representing about 62,000 workers.

Labour leader Andrew Little is a former national secretary of the EPMU.

The donations will help the two parties, which are currently in opposition, to strengthen their campaigns to change the government, says National Secretary Bill Newson.

He says only a Labour-Greens government will deliver the changes that working people need.

"We're looking forward to campaigning for a new government that is on the side of working people and our communities," he said.