The Turnbull government has been accused of hollow rhetoric on infrastructure investment after budget figures showed a projected slump in commonwealth spending by the end of the decade.

The government has committed to $75bn in infrastructure funding and finance over a decade through road and rail projects.

The program would “improve long-term productivity, increase and spread Australia’s economic growth, and deliver higher incomes for Australians”, it says.

But its yearly average spend of $7.5b is down from the $8.3b promised by the former Abbott government in 2014 through its “record $50bn infrastructure plan” over six years.

And Labor and the Greens have seized on budget figures projecting a continuous slide in infrastructure spending after a peak in 2017-18.

This is reflected in projected payments over the next five years to states for infrastructure, capital grants more broadly and financial asset investments for policy purposes, which include the likes of the national broadband network and the Northern Australia Infrastructure Facility.

The peak in infrastructure spending in 2017-18 is driven largely by investments in a second Sydney airport and Melbourne to Brisbane inland rail. The commonwealth will provide $8.4bn in equity to support the inland rail, despite a modest 1.1 cost-benefit ratio rating by Infrastructure Australia, which has listed projects such as Brisbane’s Cross River Rail as a higher priority.

Labor’s spokesman on infrastructure, Anthony Albanese, said the government had “highlighted its own failure to deliver railways, roads and other infrastructure” by revealing it would underspend by $1.6b this financial year.

Where last year’s budget set down $9.2b to support state infrastructure in 2016-17, this year’s budget showed only $7.6bn would be spent.

Alabanese said that funding “then proceeds to continue to fall off a cliff, dropping to $4.2bn in 2020-21”.

“The figures dramatically highlight the Coalition’s chronic inability to engage in the detailed proper planning that is required to roll out major projects,” he said. “They also underline the chasm between the government’s rhetoric on infrastructure investment and its actual performance.”

The Greens senator Peter Whish-Wilson pointed to other measures, including capital investment in outside the defence sector, to argue the government had “squibbed it on infrastructure funding”.

Non-defence capital spending falls “dramatically” from around $40b in 2017-18 to $23.6b in 2020-21, he said.

Its proportion of total spending was projected to then be at a 13-year low of 5.1%, with “some 40% of the government’s entire capital budget spent on submarines and frigates rather than on economy-boosting infrastructure”, Whish-Wilson said.

A breakdown of capital spending shows capital grants, which include state payments for infrastructure, will rise to $14.2b next fiscal year but drop continuously to a projected $8.2b in 2020-21.

Financial asset investments will also peak at $22.9b in 2017-18 before falling to $14.8b in 2020-21.

Whish-Wilson said the government had “failed to provide either the quantum of funding or a funding model to support a pipeline of projects that will build boost productivity”.

“We need to rapidly ramp up investment in productive infrastructure, right around the country, not just in a few select locations,” he said.

Spokeswomen for the minister for infrastructure, Darren Chester, were contacted for comment.