ACT has released a detailed proposal to consolidate the 43 existing government departments into 19.

The plan, unveiled by ACT leader David Seymour, would dismantle the Ministry of Business, Innovation and Employment and redistribute its functions across multiple new and existing agencies. "ACT said time and again it needs a trim, we have too many departments, too many ministers, too many bureaucrats," Seymour said.

Under the ACT blueprint, a new Ministry of Cities, Environment, Regions and Transport would inherit building and construction operations, Auckland functions, regional development programmes and infrastructure groups from MBIE, plus rail operations currently managed by Treasury.

A proposed Department of Home Affairs would draw immigration services from MBIE while taking over the bulk of the Department of Internal Affairs' operations and Statistics New Zealand.

The Ministry of Culture and Heritage would gain responsibility for the ministries handling Ethnic Communities, Pacific Peoples, Women and Māori Development.

A new Ministry of Commerce would combine business operations, economic growth functions, innovation programmes, workplace relations and science and technology portfolios currently dispersed within MBIE.

The proposal includes merging Justice and Corrections agencies with NZ Police, the Serious Fraud Office, the National Emergency Management Agency, Fire and Emergency New Zealand and the Office of Treaty Settlements into a single Ministry of Justice and Law.

Health services would be consolidated under a Ministry of Health and Wellbeing that would reunite the Ministry of Health with Health NZ and absorb the Cancer Control Agency and ACC.

Defence and intelligence agencies would combine into a Ministry of Defence and Security, merging the New Zealand Defence Force with the Ministry of Defence and the intelligence services NZSIS and GCSB.

The Ministry of Social Development would grow to include the Ministry for Disabled People, the Social Investment Agency, the Independent Children's Monitor, veterans services from NZDF and Whānau Ora.

Six agencies would remain unchanged: the Ministry for Primary Industries, Oranga Tamariki, the Department of Conservation, the Ministry of Foreign Affairs and Trade, the Ministry for Regulation and Treasury.

Seymour's preservation of the Ministry for Regulation drew criticism from Green Party public service spokesperson Francisco Hernandez. "It is completely telling that he has refused to actually include the Ministry for Regulation in his portfolio consolidation plan because he is desperate to protect the political fiefdom he has created for himself," Hernandez said.

Hernandez said the proposed cuts would further degrade services New Zealanders need.

The Coalition Government has official policy to reduce the size of the bureaucracy to 2017 levels by 2029. Asked whether those cuts would happen without ACT's involvement, Seymour said: "It's not as fast as ACT would do it, but does anyone think this would be happening without ACT?"

Public Service Commissioner Brian Roche has been working with public service leaders on proposals for agency-wide reform, due within the next week. "Literally in the next few months, we have to reimagine a very different operating model, and get our thinking and advice straight for the brief to the incoming government," Roche said.

The ACT plan would also affect central agencies including the Public Service Commission and Social Investment Agency.