Prime Minister Malcolm Turnbull has dismissed Opposition Leader Bill Shorten's ambitious 45 per cent emissions reduction target as heroic but unaffordable.
Mr Shorten has said Labor would go for a 45 per cent reduction on 2005 carbon emission levels by 2030.
The government has set a 26 to 28 per cent target for the same period.
Speaking in Malta, where he is attending the Commonwealth leaders' meeting, Mr Turnbull said Mr Shorten had set a high target but given no indication of how it could be achieved.
"Such analysis as exists would suggest that the cost of meeting such very high cuts in emissions are way out of line with other comparable countries.
"So it's a heroic offer that he's made but the costs would be very considerable."
Mr Turnbull said he thought Mr Shorten was making "a political rather than an environmental statement".
