Labour leader Phil Goff says use of mercenaries is "paid murder" but his leading candidate for the Mt Albert byelection, David Shearer, says the United Nations should employ them as peacekeepers.

The embarrassing divide was revealed yesterday when articles by the international aid expert Mr Shearer were dredged up by a National Party supporter.

Mr Shearer published research in 1998 and 2001 saying the United Nations should be given the option "to call on professional military companies to provide human security for a fee".

Mr Goff conversely referred to mercenary work as "paid murder" in 2003, when introducing legislation banning mercenaries.

Mr Shearer's research was highlighted by National-linked Kiwiblog website yesterday.

It was seized upon by Prime Minister John Key, who told TV3 it showed Labour's "poster boy for Mt Albert wants to privatise the Army".

Labour deputy leader Annette King said National's "dirty tricks" had begun before a candidate was even selected.

Mr Shearer is considered the one to beat out of Labour's eight nominees, with the candidate to be decided this weekend.

He has just returned from Baghdad, where he is the deputy special representative to Iraq of United Nations Secretary-General Ban Ki-moon.

Mr Shearer said his research was some of the first into the use of private security, and focused on regulating its use rather than ignoring the reality.

He said he was still supportive of using private security forces for peacekeeping as a last resort.

"If you have got a situation where thousands of people are being mutilated and it's your only option, then your first priority is the protection of women and children."

The Service and Food Workers Union's northern region secretary, Jill Ovens, said the affiliated unions would not be endorsing any particular Labour candidate.

She said her personal view was that it should be a woman, as Labour no longer had a female electorate MP in Auckland with the departure of Helen Clark.