Parliament will vote later on 13.5bn euros ($17.3bn; £10.5bn) of measures, including tax rises and pension cuts.
Greek unions are planning to mass outside parliament in protest.
Mr Samaras said without the bailout aid, the country would run out of money by 15 November and face “catastrophe”.
The fresh package of austerity measures – Greece’s fourth in three years – is meant to close Greece’s budget deficit, lower its huge debt burden and make its economy more competitive.
It includes a two-year increase in the retirement age from the current average of 65, as well as salary cuts and labour market reforms including cuts to holiday benefits, notice periods and severance pay.
Workers fear this will just make it easier and cheaper for them to be fired at a time when unemployment has already soared to 25% and a five-year recession means there are few job prospects.
The unions have staged what they described as the “mother of all strikes” – a 48-hour walkout which culminates on Wednesday evening with protests outside parliament in Athens.
The third major strike in just two months has brought public transport to a halt and shut schools, banks and government buildings.
Alexis Tsipras, leader of the left-wing opposition party Syriza, said: “The bailout policies are completely catastrophic, outrageously absurd, and an utter failure.
“Let’s not kid ourselves. The bailouts can no longer be acceptable, not even under the toughest blackmail,” he told the Efimerida Syntakton newspaper.
Wednesday’s vote on the cuts will be followed by a second vote this Sunday on Greece’s revised budget for 2013.
A positive vote on both is required for Greece to secure 31.5bn euros in fresh loans from the European Union (EU) and the International Monetary Fund (IMF).
Mr Samaras has said without this money, which will be used largely to recapitalise the country’s banks, the country will be bankrupt by the middle of the month.
However, the Democratic Left Party, which is the junior member of the three-party governing coalition, is refusing to back the austerity package.
The second biggest coalition party, the socialist Pasok, is also facing a rebellion by some of its MPs.
Despite the opposition, analysts are optimistic the fresh cuts will be approved.
Mr Samaras is believed to have the support of about 154 votes in the 300-seat parliament, assuming there are no more defections.