Labor's top three pledges to boost small business, help students, and expand research and development could cost up to $40 billion over five years.

In his budget in reply speech on Thursday, Opposition Leader Bill Shorten vowed to cut the small business tax rate by 5 per cent, write off the debts of 100,000 science, technology, engineering and maths (STEM) graduates and lift investment research and development to 3 per cent of gross domestic product by the end of the next decade.

Show me the money: Bill Shorten in Melbourne. Credit:Jesse Marlow

Business groups, the scientific community and universities welcomed Mr Shorten's focus on science and technology in his speech but questioned how Labor would find the money required to pay for the new promises.

Mr Shorten has not announced any new savings measures but pointed to Labor's plan to raise $21 billion in extra revenue over 10 years by targeting multinational tax avoidance and reducing superannuation tax concessions. Labor also argues that its new measures would enhance productivity and generate extra long-term revenue for the budget.