Finance Minister Bill English will deliver his third Budget at 2pm, amid criticisms that it will make the economic situation tougher.
The Council of Trade Unions, the Salvation Army and opposition party leaders held a rally at Parliament in opposition to cuts in social spending.
Business and media analysts were reading the Budget documents in a lock-up in the Beehive on Thursday.
Mr English will detail the cuts and changes to spending in a speech to Parliament, beginning at 2pm.
Changes already signalled include cuts in the tax incentive paid to KiwiSaver accounts, as well as cuts in Working for Families subsidies and interest-free student loans.
Union leaders, Labour and the Greens warn that more cuts to social spending and the public service will increase poverty and unemployment.
The Council of Trade Unions says Government debt is low by international standards and spending should not be cut.
But ACT says the Government needs to further rein in public spending.