Tukituki National Party candidate Lawrence Yule said this was a "massive announcement" for Hawke's Bay.
Although this expansion was to enable future growth, Mr Yule said its effects would also be felt in the more immediate future by easing traffic congestion - which is expected to get worse when the Whakatu arterial link is completed.
Although the cost of expressway improvements had not been worked out yet, Mr Yule said it was thought to be in the "hundreds of millions".
As well as a lane expansion, these roads typically also include wide lanes and wide, sealed shoulders, and mainly grade-separated intersections - which means they mainly have over bridges, underpasses or well-spaced at-grade intersections.
At the launch, Transport Minister Simon Bridges said the original seven Roads of National Significance - which were either complete or under construction - were enabling better freight movements, easing commute times, making roads safer, and supporting New Zealand's economic growth.
The road between Hastings and Napier would help "gets the produce to the port, and means for this growing population much better, safer travelling".
The Manawatu Gorge was also designated a road of national significance.
"We will have a preferred new route by the end of the year," Mr Bridges said. "By making it a road of national significance will give everyone the confidence of its importance and the certainty and the priority it is getting."
These projects were the highest volume roads, and would "help provide a strong, safe highway network that links our regions effectively with our major cities".
The new roads were expected to cost around $10.5b, on top of the estimated $12b invested in the initial seven.
They would be funded from the National Land Transport Fund and the use of public-private partnerships.
The initial funding would come from the infrastructure investment of $32.5b in Budget 2017.
Mr Bridges said it had not been decided which order the roading improvements would be implemented in, but thought the Hawke's Bay expressway could be completed quickly.
It was envisioned as running from just north of Hastings through to Hawke's Bay Airport.
The other nine roads of national significance covered areas from Whangarei to Christchurch, including Cambridge to Tirau, Tauranga to Katikati, and the Manawatu Gorge.