Labor and the Greens would have the numbers to block any government proposals in the upper house had proposed Senate voting reforms been in place in the past two elections, according to research by the Parliamentary Library.
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Parliament's Joint Standing Committee on Electoral Matters last year recommended the introduction of optional preferential above the line voting, which would allow voters to number as many boxes above the line as they wished.
Under the current system, micro-parties can win seats despite attracting only minuscule shares of the primary vote due to the group voting tickets that parties lodge with the Australian Electoral Commission, which dictate the flow of preferences when a person votes above the line.
Optional preferential above the line voting would allow voters to control where their preferences go without numbering every box below the line as they currently must.
While the proposal had the unanimous support of the committee, which included Coalition, Labor and Green MPs, Labor Senator Sam Dastyari last month publicly argued against the proposal on the grounds it would disadvantage progressive candidates.
However modelling produced by the Parliamentary Library for the Greens shows if the policy had been in place for the 2010 and 2013 elections, the ALP and the Greens would between them have 41 seats, enough to veto any Coalition proposal.
In the current Senate the two left-of-centre parties have only 35 seats, meaning they need the votes of three crossbench senators to block legislation.
The modelling is based on the assumption that no preferences are allocated, as it is difficult to predict how voters would choose to direct their preferences.
But based on first preferences, at the 2013 election the ALP would have won two extra seats, while the Coalition and the Nick Xenophon group would have won an extra seat each.
These seats would have come at the expense of Jacqui Lambie, who was elected on the Palmer United Party ticket in Tasmania, South Australian Family First Senator Bob Day, Victorian Senator Ricky Muir of the Australian Motoring Enthusiast Party and Australian Sports Party candidate Wayne Dropulich.
Mr Dropulich originally won a WA Senate spot but lost it to Palmer United Party Senator Zhenya "Dio" Wang in the 2014 WA Senate byelection.
The Greens would have gained a Senate spot in NSW but lost Sarah Hanson-Young's spot in South Australia.
Based on first preferences in the 2010 election, Labor would have gained two Senate seats, at the expense of the Coalition and the Democratic Labor Party.
Senators Lambie and Xenophon along with Liberal Democratic Party Senator David Leyonhjelm will debate Senate voting reform at a forum at the National Press Club on Wednesday.
