Accusations of free-loading and the "poaching" of organised labour has seen spat between unions boil over into the public domain.

As the bulk of New Zealand's junior doctors finish a 48-hour strike to fight for better working conditions and safer rostered hours, an email has emerged in which the head of the Public Service Association (PSA) has taken aim at the union representing them.

A particular sore point appears to be that the Resident Doctor's Association (RDA) is not an affiliate of the Council of Trade Unions (CTU), and also uses a corporate bargaining agent as its advocate in negotiations.

In the email, leaked to Stuff, it is unclear exactly what PSA Secretary Erin Polaczuk was responding to in reply to counterpart Ian Powell at the Association of Salaried Medical specialists, but she makes clear her view of the RDA's position within the wider union movement.

"Firstly, it's not 'irrelevant' that the RDA is not an affiliate. The CTU has a particular relationship and responsibility to CTU affiliates that should not be automatically granted to any free loading non affiliate.

"And of course, the RDA is not just any union," Polaczuk writes.

The ASMS represents senior doctors and is a CTU affiliate, while the RDA represents the vast majority of resident, or "junior", doctors still completing their training, and is not.

JOHN KIRK-ANDERSON/STUFF Junior doctors strike at Christchurch Hospital.

RDA secretary Deborah Powell is also the owner of CNS - the corporate bargaining agent contracted to act on behalf of the RDA and the union which represents corporate and executive employees in the health industry (Apex).

It's a model Polaczuk has described as "anathema" to the PSA and CTU, because CNS is "motivated by raising profit from representing workers".

The PSA and Apex are also locked in a competition for membership and Polaczuk goes on to accuse CNS organisations of trying to steal its members. However, the RDA has accused the PSA of using a new break-away union to undermine the RDA's legitimacy.

"The fact that workers are leaving CNS across the board should come as no surprise given their infamy, modus operandi and history. PSA strongly supports the CTU code of operating between affiliate unions and disagrees entirely with the approach that CNS organisations take, which is to poach organised labour," Polaczuk wrote.

The email was written in September. At the time, it's understood the PSA was offering to provide industrial support to the breakaway doctors union Specialty Trainees of New Zealand - or STONZ.

Heath Lash, who heads STONZ, said his organisation had paid for initial support from the PSA, which mainly consisted of advice in setting up its website and members database.

NZRDA Dr Deborah Powell, national secretary for the New Zealand Resident Doctor's Association.

The PSA also provided an expert to offer procedural advice if called upon, while STONZ negotiated its own agreement with District Health Boards (DHBs).

STONZ negotiated its own two-year MECA (Multi-Employer Collective Agreement) with DHBs at the end of last year, while the RDA agreement - which included a "safer hours roster" - had expired.

That meant there could be more at stake for the RDA.

The RDA had until February to negotiate a new agreement, otherwise it would no longer be able to accept new members and DHBs will only be able to offer the STONZ collective agreement to new doctors, or individual employment agreements.

The PSA declined to comment.

Deborah Powell rejected a number of Polaczuk's claims in the email, including that the RDA had "consistently rejected" CTU affiliation.

"That's just rubbish. RDA is not affiliated to anyone, we are independent. In fact the RDA has always taken the position that their only master can be their members."

The union worked "closely and constructively" with a number of other unions, as well as on a national partnership group between unions and DHBs.

ABIGAIL DOUGHERTY/STUFF Junior Doctors on strike outside Auckland Hospital calling for safer working conditions.

Deborah Powell also rejected the PSA claim that its inclusion on that group was "accidental".

"This is PSA hierarchy, which is incredibly out of touch with what's going on, on the ground."

She said STONZ "put their training ahead of safety".

"The PSA was supporting them. The rest of the trade union movement did not believe the PSA should because they perceived STONZ to be an employer's union - too close to the employer.

"The problem with unions that are too close to the employer is that it leads to employment agreements that disadvantage employees. That's exactly what's happened. STONZ has settled a collective agreement with the District Health Boards, with vastly inferior terms and conditions to that of the RDA collective."

Deborah Powell said that as well as clawing back an agreement for safer working hours with DHBs, the STONZ agreement had reduced sick leave, reduced access to reimbursements for the costs of training and reduced access to house surgeons to protect teaching time.

STONZ secretary Heath Lash said he had no knowledge of the background of the email, but his union was intending to join the CTU some time in the future and understood some trade unionists might have a problem with that.

He accepted the formation of STONZ had "changed the landscape" for the RDA. But he defended the need to break away from a contract which did not suit the most experienced junior doctors, who had already been in the profession for up to 13 years.

"It's a big group of people - it's very diverse and we need different things at different parts of our careers. That's why STONZ was formed, because the RDA contract didn't really fit for those people like myself who had a pathway for training to becoming specialist."

ASMS head Ian Powell, (no relation to Deborah Powell), would not comment on issues raised in the email. However, he said the language was "unfortunate".

"It makes attacks that are not substantiated, it implies that doctors in training are like lambs without a mind of their own.

"A fair proportion of our members are former RDA members and there's an implication there that our members have graduated from lambs to sheep without a mind of their own."

CTU President Richard Wagstaff has been contacted for comment.

Despite the PSA's position, the junior doctors had support from a number of other CTU unions including the ASMS and the New Zealand Nurses Organisation.

NZNO Industrial Advisor Lesley Harry said resident doctors deserved better than having their conditions clawed back.

"This is completely unacceptable when we are currently facing a serious shortage of RMOs and medical specialists across the board resulting in inadequate and unsafe staffing levels.

"Demand on hospital services continues to increase, which means we need to be forward thinking. We should be attracting and retaining trainee doctors and treating them with respect because better staffing levels are vital in sustaining health services."