Nationals leader Barnaby Joyce says he wants federal government "largesse" to be spread more evenly around the nation but Labor has re-doubled its attack on the policy, saying the library's research is clear proof of the "pork barrel".

The library has found only four sites in nation fitting the bill under the government's new rules for relocating agricultural agencies, written at Mr Joyce's behest.

Three of the towns are in Nationals' seats and a forth in a north Queensland marginal electorate the party desperately wants to win back.

The Location Corporate Commonwealth Entities policy order, drawn up to make the controversial move of the pesticides authority, the APVMA, from Canberra to Armidale binding, mandates where agricultural agencies should moving out of Canberra should locate their headquarters.

But under the order, struggle states Tasmania and South Australia could get nothing, despite Mr Joyce's rhetoric about spreading "largesse" around the nation, nor will Victoria, Western Australia or the Northern Territory.

Mr Joyce rejects the criticism arguing that he is also moving public servants to other regional centres around Australia.

The key site, Armidale in the heart of Mr Joyce's own New England, is to be the new home of the national pesticides authority.

Another two sites identified by the library as eligible for agency HQs, Rockhampton and possibly Bathurst are firmly in Nationals or LNP-held territory.

The fourth, Townsville, is in the heart of the marginal seat of Herbert, won by Labor in 2016 which the Liberal-National Party would dearly love to win back.

The policy order states corporate Commonwealth entity with agricultural policy or regulatory responsibilities is to be located "In a regional community, within 10 kilometres by road of the main campus of a regional university involved research agricultural science."

"Regional communities" must be at least a 150 kilometres drive from Canberra or a capital city of a state.

The rules leave the LNP well set-up for a pre-election promise to move a Commonwealth agency to Townsville, which is struggling with unemployment, a move that has worked in the 2013 election in both Mr Joyce's seat and in Robertson in NSW.

Labor's Agriculture spokesman Joel Fitzgibbon, who commissioned the Parliamentary Library research, says the map conclusively proves the decentralisation push is a "pork barrelling exercise from the beginning."

"Barnaby Joyce designed a policy order to ensure that Armidale was eligible," Mr Fitzgibbon said.

"What he needs to explain, amongst many other things, is why the other three towns that are eligible weren't afforded an opportunity to make a bid."

The Labor frontbencher said the location of the eligible towns in seats held or targeted by the Coalition was no surprise.

"It is hardly surprising that three of the four are Coalition held seats and the fourth is one they would dearly like to hold and would hope to win back at the next election.

"That's no doubt by design."

A spokesperson for Mr Joyce said regional Australia deserved "its fair share of government services and opportunities; and that includes being able to access quality public sector careers just as much as any capital city.""That is why we are moving the APVMA to Armidale; the Grains Research and Development Corporation \has opened offices in Toowoomba, Dubbo, Adelaide, and Perth; the Rural Industries RDC has moved to Wagga Wagga; and the Murray-Darling Basin Authority is opening offices in Toowoomba, Albury-Wodonga and Adelaide," she said.

The main public sector union, the CPSU, said the policy would only benefit "a tiny fraction of the people seeking quality jobs in regional Australia."

"The APVMA relocation is causing real damage to the fundamental functions of the agency but also does nothing to address the real need for more public sector jobs outside the metropolitan centres."

"The so-called benefits to Townsville for example would be a handful of jobs through a possibly damaging and illogical relocation when the Turnbull Government has cut far more positions in and around the city in recent years."