Tony Abbott is the true "inter-generational thief" for boosting fossil fuels at the expense of renewable energy, increasing the likelihood of Australians paying a big price from climate change, Greens leader Christine Milne said.

Both the Prime Minister and Treasurer Joe Hockey have based their budget sales pitch on the need to raise taxes and slash spending to rein-in future debt that had been projected to balloon above $600 billion without policy changes.

"We could not go on running up massive debts for our children and grandchildren to pay," Mr Abbott told ABC's Insiders on Sunday. "That would be a form of inter-generational theft."

Senator Milne, though, said the increases in subsidies for big miners – which rise by $720 million to almost $14 billion over the forward estimates – and efforts to promote more coal seam gas, coal mines and coal ports were the real form of "future theft".