Almost one third of sitting MPs worked mainly in politics before entering Parliament, a new study reveals.

The University of Otago study, undertaken by politics researcher Geoffrey Miller and the brainchild of PR director and political strategist Mark Blackham, looked at what the main careers of 120 sitting MPs were before they entered Parliament.

It found up to a third had political backgrounds, which Mr Blackham said showed an era of 'career politicians' ushering in "a new era of dirty politics".

"Politics is now a professionalised game, with a major speciality being digging dirt," he said.

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"Politics has always been a dog fight, but underhand denigration of your opponent is now considered part of the job description; it makes up for absence of real policy differences.

"Career politicians find it easier to be against something than for it -- so it is essential to demonise your opponents."

The previous careers of MPs don't vary too much from party to party, the study said, with 18 per cent of National MPs having a background in business, and 12 per cent of Labour MPs.

"We're seeing people enter Parliament because of their own self-interest, not their life experience," Mr Miller said.

"In the past, the differences between parties were starker. For example, there were more businesspeople in National, and almost none in Labour."

However, a merging of policies was forcing parties to focus on the personalities, he said.

"Labour and National are now almost interchangeable in terms of policy, which is why some are keen to distract voters with scandal," Mr Miller said.

"In 2014, it would be difficult to imagine National repeating its famous 2005 billboard campaign in which it directly contrasted its policies with Labour's, because there is now so little difference.

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"Working for Families, KiwiSaver and subsidising first-time home buyers are all examples of policies which Labour and National now largely agree on."

Key findings:

* 33 per cent of MPs have worked in a career which could be seen as political before entering Parliament.

* Work experience only in government is the single most common career -- right across all parties: 15 per cent of MPs have worked as a civil servant, parliamentary staffer, or in local government; 11 per cent have only ever worked in this category.

* National MPs had the most experience in agriculture and business or property development with 12 MPs in each category.

* Politics and education are the two main employment categories of Labour MPs.

* National is stronger on health backgrounds than Labour (six MPs vs three), and in legal experience (six MPs vs two). Labour's two lawyers (David Parker and Raymond Huo) have both had prominent secondary careers.

* Labour MPs were most likely to have had no single dominant career, or to have worked for government in some way.

* New Zealand First has proportionately the most MPs with business backgrounds, especially small business (three from seven MPs).

* The dominant working history of Green Party MPs is in unions or activist agencies (six MPs).

* National MPs have had the broadest range of work experience. This was partly a by-product of having the most number of MPs, but was consistent when considered proportionately.

* No single career path is dominant to a statistically significant degree, in any party.