Prime Minister Tony Abbott said the budget was ''striving to be fair'' as he criticised modelling showing the the poorest 20 per cent of families will be hit much harder than the wealthiest households, saying the assumptions behind it were devised by the opposition.

The accusation was rejected by the National Centre for Social and Economic Modelling (NATSEM), which said the modelling was commissioned by Labor but the analysis used its standard and long-standing model for distributional impacts.

While saying NATSEM's modelling was ''not pure'', Mr Abbott did not explicitly dispute its central finding revealed by Fairfax Media - that the poorest 20 per cent of households would contribute $2.9 billion over four years to the government's fiscal repair effort, while the richest 20 per cent would contribute $1.8 billion.