Consultants and contractors are a favourite punching bag for opposition parties - of all colours - and National is just the latest to land a solid blow, leaving Labour off-balance.

"Under National, this gravy train's gonna stop at the station" - Christopher Luxon

Consultants and contractors are a favourite punching bag for opposition parties - of all colours - and National is just the latest to land a solid blow, leaving Labour off-balance.

The numbers show public service spending on consultants and contractors has recently surged, and the prime minister has largely left it to the sector's watchdog to mount a defence..

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When Labour took office in 2017, the annual spend on consultants across the core public service - 35 ministries and departments - was about $900 million.

At the time, Chris Hipkins as State Services Minister was describing the situation as unsustainable: a contracting blow-out he blamed on National.

He pledged to cut back that spending - and it did level off for a time - but in the last year the figure has surged, topping $1.2 billion in the latest update.

National's leader Christopher Luxon, in his state of the nation address last Sunday, paired an early childhood education rebate policy with a $400m cost-saving plan to cut spending on consultants and contractors.

Luxon and his party have been critical of the public sector's spending on consultants and contractors.

Crunching the numbers, the party added in spending by Crown entities like Waka Kotahi and Kāinga Ora not included in core public service numbers, calculating a total bill of about $1.7 billion.

Either way it's up by about a third on like-for-like numbers - a sizeable increase. National reckons it can wind back the clock, returning the spend much closer to those 2017 levels.

National has been relentless in its attack, pursuing the subject every day in Parliament's debating chamber, select committee meetings, and in media interviews. The focus is strategic: National sees the public service as a vulnerability for Labour and for Hipkins especially.

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Go to this episode on rnz.co.nz for more details