Liberal frontbencher Bronwyn Bishop has backed reported calls from some in the Liberal Party for an end to the system where voters have to number all the candidates in the lower house when they vote.

This would mean that voters would only have to place a ''1'' against a candidate to cast a valid vote, instead of the current system which requires voters to number all the candidates in their electorate.

On Thursday, Mrs Bishop - who is shadow special minister of state - said that compulsory preferential voting had led to a rise in informal votes. ''To have an optional preferential vote means that you would increase the valid votes that are cast,'' she told ABC radio.

''There are many people who object to being forced to vote for somebody who is someone they simply don't want to cast a vote for but they still wish to vote.''

According to the Australian Electoral Commission, the 2010 federal election had the highest number of lower house informal votes since 1984, with a rate of 5.55 per cent.

There is already optional preferential voting in New South Wales and Queensland state elections.

A move to a voluntary system could also boost the Coalition's vote. ABC election analyst Antony Green has noted that such a system would likely hurt Labor's overall vote, which relies more on preferences.

But on Thursday, Mr Truss - who is leader of the Nationals - stamped out the idea while visiting fire-affected areas of Tasmania.

''There will always be healthy debate about these issues, but the Coalition has no plans or policy to change the current system of compulsory preferential voting,'' he told Fairfax Media.

The Nationals have a history of opposing optional preferential voting, as it makes it harder to compete in a three-corner battle against the Liberal Party.

Nationals Senate leader Barnaby Joyce has cautioned against the proposed change.

Special Minister of State Gary Gray said that compulsory and full preferential voting had been in place for nearly a century and should not be changed lightly.

''The latest proposal is characterised as desirable because it provides a partisan political advantage,'' Mr Gray said in a statement. ''That is wrong.''

Last week, the Queensland Liberal-National Party government released a discussion paper on planned electoral reforms, which included a section on possibly ending mandatory voting.

The idea was panned by the federal Labor government, with Prime Minister Julia Gillard accusing the Liberal Party of trying to make democracy ''the plaything of cashed-up interest groups''.