Parts of a $3.8 billion plan to bring forward infrastructure projects by the Morrison government won't be spent until the second half of next decade amid concerns the stimulus will be too little to prevent the Reserve Bank using unconventional policies to boost the economy.

As a monthly measure of job advertisements showed further slowdowns in the key NSW and Victorian markets, the government talked up the impact of its package of road and rail projects alongside its income tax cuts and assistance for drought-hit parts of the country.

Doubts have been raised that the Morrison government's $3.8 billion infrastructure package will be large enough to prevent a further fall in interest rates. Credit:Sam Mooy

The government is bringing forward $2.7 billion in planned projects and topping that up with an additional $1.1 billion, striking agreements with state governments to get works under way.

Some of the money will be spent in coming months, such as $225 million on a string of Bruce Highway upgrades in Queensland, but in some cases the money will be spread out over five or six years or the project itself won't commence until 2021-22.