The deterioration comes after a week in which Prime Minister Tony Abbott announced the terms of reference for a wide-ranging and potentially open ended royal commission into union corruption, naming five unions specifically - one of which was the AWU, the union giant formerly run by the Labor leader. Labor's share of the primary vote has also fallen by 4 points to be just 33 per cent - the same disastrous level Labor achieved in the 2013 election in which it was roundly defeated. On a two-party-preferred basis, the Coalition now leads the ALP by four percentage points - 52 per cent to 48, according to the allocation of preferences at the last election. However, that gap narrowed to 51/49 when voters were specifically asked who would get their second preference. The poll surveyed 1400 electors between Thursday and Saturday. It found the Coalition would have been returned strongly had an election occurred at the weekend. However, with growing anxiety over job losses and a fragile economy, the Coalition's two-party support remains 1.5 per cent lower than its share at the September 7 election.

Mr Shorten, who favours the more focused use of existing law enforcement authorities to address union corruption, strongly opposed the royal commission, prompting Mr Abbott to accuse him of ''running a protection racket for a protection racket''. Falling in the leadership approval poll: Opposition Leader Bill Shorten. Credit:AAP The poll suggests that line may have cut through, with a clear majority of all voters believing there is a case for a royal commission into corruption within unions. Even 55 per cent of Labor voters back the decision. Only 23 per cent of voters oppose the union inquiry, a figure that climbs only modestly to 36 per cent among ALP voters. The poll has a margin of error of plus or minus 2.6 per cent, making Labor's four-point drop, statistically significant.

Support for the Greens has climbed 3 points since the September election to 12 per cent. On primary votes, Labor's 33 per cent compares with the Coalition's 44 per cent. As the incumbent prime minister, Mr Abbott enjoys a 10 point lead over the Opposition Leader as preferred prime minister, 49 per cent to 39 per cent. But on personal approval, voters appear to be marking Mr Shorten down for his past career as one of the country's most prominent union leaders. The number of people approving of his performance fell from 51 per cent in the Fairfax-Nielsen poll of November 21-23, to be just 40 per cent now - the same proportion coincidentally who disapprove, giving him a net approval rating of zero. By contrast, Mr Abbott's approval rating has fallen by a statistically insignificant 2 points to be 45 compared to 47 per cent of respondents who disapprove of the way he does his job, for a net approval rating of -2 per cent.

While Australians appear unsympathetic to unions themselves, they are happy enough to pocket the gains made by unions and successfully defended by them over decades. Asked if they agreed that ''workers' entitlements and conditions need to be reduced to make Australian companies more competitive'', 58 per cent said no. That proportion climbed to 76 per cent among Labor voters, and 81 per cent among Greens voters. Loading The Coalition government had dropped behind Labor in the Fairfax-Nielsen poll in November breaking a record in the 40-year history of the survey for being the fastest slump to a losing position by a newly elected government.