Roadworks on the Brynderwyn Hills are already $3 million over budget, Northland MP Winston Peters says, claiming the blowout will rise and put at risk other vital roading projects in the region.

The New Zealand Transport Agency began the project - said then to cost $15.9 million - in early December 2014 and expected to have the entire project completed by November this year.

In Parliament yesterday, Associate Minister of Transport Craig Foss said the cost had risen from $15.9 million to $18.9 million and he blamed "new information" on geotech issues. Northland MP and New Zealand First Leader Mr Peters said the blowout would likely see desperately needed roading projects, including bridges, in Northland short-changed, or abandoned.

The Brynderwyns improvements continue to cost more each month, he said.

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"Northlanders should rightly be able to question why these (geotech) issues weren't allowed for in the initial planning and budget," Mr Peters said.

"More importantly, the Government must assure them that other projects will not be adversely affected.

"Mr Foss suggested that this would not be the case, but this was not reassuring given their by-election promises.

"The truth is the Government has yet to show where they put the $69 million, the amount they promised to spend on 10 new two-lane bridges - none of which have been built and only four are on the project books."

But Brett Gliddon, New Zealand Transport Agency's Northland and Auckland highway manager said the extra costs would not impact on other work.

"The original budget for the Brynderwyn Improvements was $15.2 million with an initial allocation including contingencies of $17 million. An additional $1.9 million is required in response to unforeseen geotechnical issues on site," Mr Giddins said.

"The additional spend can be accommodated within the Transport Agency's State Highway Allocation and we don't believe that will have an impact on any other priority projects in the area."

The Brynderwyn work includes widening State Highway 1 (though not extending or putting in new passing lanes); removing tight corners and installing a median wire rope barrier to separate north and south-bound traffic.

It will also provide a place for people to stop and admire the views from atop the hill, with a new look-out over Bream Bay, Hen and Chicken Islands and the Waipu Plains being constructed.