Ms Ardern's government announced $NZ8 billion ($A7.7 billion) for roads, rail, schools and hospitals on Wednesday, part of a $NZ12 billion package.

Around half of the cash splash will go to Auckland, the country's biggest and most congested city, home to roughly a third of Kiwis.

Major new road links north and south of Auckland have been funded, as well as a new five-kilometre walking and cycling link over the Auckland Harbour Bridge at the eye-popping cost of $NZ360 million.

Another $NZ940 million will go two road projects near Tauranga.

Less than $NZ400 million - or five per cent - will be spent on South Island.

"This is a once in a lifetime opportunity to invest in New Zealand," Ms Ardern said.

The government believes up to 9000 jobs will be created due to the spend, prompting Ms Ardern to make this pitch to young Kiwis.

"There has never been a better time to enrol in a trade. I'll say that again. There has never been a better time to enrol in a trade," she said.

"There are good jobs that pay good money in all these projects ... young New Zealanders should consider taking up a building or other trade apprenticeship."

Road projects amount to around $NZ5.5 billion, with rail projects funded with $NZ1.1 billion.

Three new acute mental health facilities are included in $NZ300 million worth of capital investment in health, with $NZ200 million set aside for decarbonising power sources to public facilities.

Finance minister Grant Robertson - who will oversee the program - said the spending was affordable.

"It makes sense to take advantage of record low long-term interest rates and invest in infrastructure to grow the economy," he said.

"We have the room to do this because we have managed the books carefully.

"International credit ratings ... have backed our decision to invest in more infrastructure."

The announcements kick off a year of election campaigning after the prime minister announced on Tuesday that Kiwis will head to the polls on September 19.

While Labour leaders Ms Ardern and Mr Robertson steered clear of any references to the opposition National party or the election, Deputy Prime Minister Winston Peters played the role of attack dog.

Mr Peters said Opposition Leader Simon Bridges, who was transport minister during National's last term in government, was guilty of talking "bovine scatology" on his record of funding transport projects.

Despite the $NZ200 million decarbonisation program, which will see the replacement of coal boilers at regional schools and hospitals, Greenpeace said the spend was a missed chance to "clean up New Zealand's transport system".

"In New Zealand, we have particularly inefficient cars, and increasing their use will only contribute to more of the dirty emissions that are driving the climate emergency," campaigner Amanda Larsson said.

Australian Associated Press