Much deeper reductions in Australia's carbon emissions are not only justified and desirable, but also eminently affordable, a report by the government's independent climate adviser has found.

The report from the Climate Change Authority found the current target of a 5 per cent cut in carbon was ''inadequate'', and noted even that minimum goal would be more cheaply and easily achieved by altering the Coalition's $3.2 billion direct action plan.

The authority recommended a higher target of a 15 or 25 per cent reduction in carbon emissions by 2020, saying conditions laid down by both major parties for deeper cuts had been met as more nations had adopted more ambitious goals.

''The costs of moving beyond 5 per cent are relatively small, and moving beyond 5 would put us more in line with what other countries are doing,'' the authority's chief executive, Anthea Harris, said.

Based on growth projections, the cost per person of a 15 per cent target would be just $100 in 2020. A 25 per cent cut would cost a further $100 by then.