The Coalition has ruled out amending a contentious Senate voting reform bill to allow voters to number fewer than all the boxes “below the line”.

The Greens, whose support along with Nick Xenophon’s could see the voting changes passed in time for a July double-dissolution election, have indicated they will not push for the amendment, even though it is long-standing party policy.

The Family First party senator Bob Day on Monday joined calls by constitutional experts and psephologists to ditch full preferential voting below the line, the current system that demands voters number every box or risk spoiling their vote.

“If we were to accept the argument that voters do not understand the system, then the solution is to make it less onerous below the line,” he said in a submission.

He said voters should have to number only “so many boxes as deemed necessary”, suggesting six in a regular half-Senate election – in line with the number of seats contested in each state – or 12 in the event of a double-dissolution.

The current bill focuses on “above the line” voting, proposing to abolish the system of group voting tickets in which voters can mark 1 in a single box and have their preferences flow in line with party agreements.

Instead it would enshrine optional preferential voting, where voters number their preferred parties from first onwards, with “advice” that electors number at least six boxes.

Bolstered “savings provisions” would be introduced below the line, allowing voters to make up to five sequencing errors without spoiling their vote, up from the current limit of three.

Prof George Williams, an expert in constitutional law at the University of NSW, has argued the changes are a major improvement on the current system but said that maintaining full preferential voting below the line “[enables] parties to affect the result in a way that is not a true reflection of voter preferences”.

“Disturbingly, it would do this in a way that would create the impression that this bill is designed to harm the electoral chances of minor parties while retaining the capacity of major parties to manipulate the preferences of voters through the ordering of candidates,” he said in a submission.

Psephologists Peter Brent and Antony Green have also argued for optional preferential voting below the line, which was a recommendation of the 2014 Senate inquiry into the voting system.

Green said it was “disappointing” the legislation did not introduce optional preferential voting below the line, arguing the same expression of preferences could be considered formal above the line, but informal below.

It would also “allow voters to optionally express preferences for candidates in the order presented by parties, but deny voters the option to optional give preferences for the same candidates in a different order,” he said in a submission.

“It is the sort of inconsistency that attracts the attention of the High Court, especially when Section 7 of the Constitution states that Senators should be ‘directly elected by the people’.”

But the finance minister, Mathias Cormann, told Guardian Australia on Monday the government was “not proposing to make changes to below the line voting” other than the new savings provisions.

He said that by 2013 federal election figures the proposed improvements to the savings provisions “would have saved about 45% of informal below the line votes”.

The Greens senator Lee Rhiannon said optional preferential voting below the line, to a minimum 12 places, remained her party’s position but did not say whether she would push for a specific amendment in that vein.

“We will continue to work to improve the bill to increase the ability of voters to decide their own preferences either above or below the line,” she said.

The changes have been the subject of bitter debate in the Senate, with the Labor senator Penny Wong on Monday arguing the Greens’ support for the bill was embarrassing and “completely unjustifiable”.



She criticised the four-hour hearing on the bill scheduled for Tuesday, calling it a “half-day sham inquiry”.

“We just don’t think you’ve got the right answer and we don’t think ramming it through the Senate ... is the right thing to do either,” Wong said.

Another Labor senator, Doug Cameron, said the Greens and the Coalition were “at one in destroying democracy in this country”.

The Greens leader, Richard Di Natale, said Senate reform had been the subject of a thorough inquiry in 2014.

“It is laughable that the ALP would support an arrangement that would continue to entrench power in the hands of factional operators but would not give power back to voters,” he said.

Labor have sought to cleave the Greens from the Coalition by amending the bill to improve political donation laws, including by reducing the disclosure threshold to $1,000 and publishing donations records more quickly.

Rhiannon said the Greens had a long-standing commitment to changing political donation laws but had not seen any amendments. “The party room will make a decision when we do,” she said.

Around 481,000 votes were cast below the line at the 2013 federal election.