"We will be encouraging our partners, and supporting their efforts, to pursue a career beyond MFAT," the letter states.
Diplomats' families would be discriminated against in the new ministry structure, the group of partners said.
"We have travelled and served in inhospitable and insecure environments, accepted disruption to careers and schooling, absorbed loss of income and pension, and felt the impacts of long absences from family and friends.
"The proposed changes to career and remuneration could end up discriminating against families, providing us with little reason to continue making the personal, financial and professional contributions and sacrifices for MFAT and New Zealand's international reputation."
Bronwen Golder, the group's spokesperson, said many partners of diplomats had made significant sacrifices and felt disrespected.
"Diplomats' partners and families feel insulted by the proposals.
"It's already hard to convince MFAT partners to move overseas. The proposed changes are so significant that many MFAT couples will not consider another posting for MFAT. That will represent a great loss of MFAT talent, experience and diversity. "
The MFAT partners group polled 83 of its members over an eight day period about their earning potential, the PSA said.
All had been in full time paid employment before their partner took up a diplomatic posting and all but one had since had to resign from their position, it said.
Mr Allen has admitted the $25 million raft of cuts will only go halfway to achieving the ministry's full savings targets.
Under questioning from Labour's foreign affairs spokesman Phil Goff on May 1, he admitted the full target for savings was $40 million a year.
