Their criticism of the Commonwealth Electoral Amendment Bill - designed to end the era of "preference harvesting" by micro parties - centred on a perceived discrepancy in that the new Senate ballot paper will provide for optional preferential voting above the line but retain full preferential voting below the line.

Lee Rhiannon is in open conflict with former party leader Bob Brown Credit:Andrew Meares

That means that if a voter made the same six preferences below the line as they did above the line their ballot would be deemed invalid because those going below the line will still have to number all boxes - which can entail choosing between more than 100 candidates in some states.

Professor Williams described the proposed new system as "incoherent" and Mr Green, who broadly supports the changes, which he said would "put preferences back in the hands of voters" said the anomaly above and below the line were "a foot in the High Court challenge".

Mr Mackerras believes the proposed system is unconstitutional because it does not adhere to the "candidate-based" system envisaged at federation.