The transport agency is already working to do this on selected highways in other parts of New Zealand, but only with one or two changes in a region.
MacMillan said Northland was 'one of the challenging ones that if we can crack it, it will be good."
She said it was not financially possible to engineer major road safety improvements such as median barriers for every state highway in rural areas such as Northland.
Speed limit was a more immediate and less expensive way to address road deaths and significant injury.
Northland has six state highways stretching 880 kilometres along Highways 1, 10, 11, 12, 14 and 15. MacMillan said 770km of Northland's state highways had been earmarked for the speed reduction. She would not be drawn on whether the speed reduction would refer to the whole region in a blanket approach.
Waka Kotahi reduced the Kawakawa to Moerewa SH1 limit from 100km/h to 80km/h in August.
Northlanders are expected to 'soon' be able to have their say on the Waka Kotahi proposals for the region.
Waka Kotahi was at edition time unable to provide a timeframe for this in terms of the consultation start date. It was also unable to indicate when the proposal on which the community consultation would be based would be finalised.
Waka Kotahi is looking to permanently slash Northland's state highway speed limits as part of Road to Zero, a national safety campaign to reduce road accidents deaths and injury.
Cutting the 100km/h speed limit on the region's state highway network to 80km/h would be the biggest Northland main road speed change for almost 40 years.
New Zealand's national speed limit was raised from 80km/h to 100km/h in July 1986. It had been at 80km/h for 13 years before then - after being reduced to that speed in December 1973 as a fuel-saving measure.
The proposed reduction has angered some Northland roading leaders who have said its potential blanket approach was not the best way to address the problem and warned slashing mandatory 100km/h speed limits must not be used as an excuse to put less government money into Northland state highways safety improvements.
MacMillan said Northland roading leaders were an important part of the group approach needed to make the proposal work and reduce death and serious road injury.
She said there had been unacceptable trends in state highway road deaths and injuries in Northland. That needed to be stopped.
Everybody needed to play their part in everyone getting home safely and reversing these trends.
MacMillan said Waka Kotahi would take consultation feedback into consideration and in some situations had modified its approach to speed limit reductions for state highways in other parts of New Zealand.
She was unable, however, to provide an example of where this had occurred on the ground.
