The announcement was made "out of the blue", he said.
The union had been told there was no timeline for a decision to be made, he said.
Mr Gallagher said those who will lose jobs will find it hard to get other employment in the current economic times.
Labour Party leader David Cunliffe said the decision to cut jobs in Rotorua would have a huge impact in the city where 130 jobs were lost last week from the Tachikawa Mill receivership.
"It will be very hard for these workers in a tough job market."
Mr Cunliffe said email growth had put the growth of NZ Post's letter business under great pressure.
"It will have a huge impact in the provinces where already over 200 jobs have been lost in the past month in Shannon and Rotorua.
"It is hard to see how provincial New Zealand can absorb these large numbers of unemployed.
The "hands-off" approach from the current government has failed to create the 170,000 jobs it had promised in 2011, he said.
Prime Minister John Key said the announcement was not made "out of the blue".
"There's been a lot of discussion about what would happen if there was a change to the number of days that the mail was delivered to people's homes," he said.