SYDNEY - Australia's national election has become too close to call, polls out on Wednesday showed, as the ruling conservative coalition narrowed the gap with the main opposition Labor Party, three days before the country decides on a new government.

Centre-left Labor's lead over the Liberal-National coalition has shrunk to 51-49% on a two-party preferred basis from 54-46% two weeks ago, a poll for the Sydney Morning Herald showed, while a Guardian poll indicated it had dipped a point to 48%.

Prime Minister Scott Morrison described the pre-polling trends as "really encouraging".

Nearly 6 million voters out of an electorate of 17 million have already cast their ballots through postal votes or early in-person voting, official data showed.

An additional 1.1 million postal votes have been received so far versus the 2019 election.

"If it's close, this level of postal votes makes an election night indication of who forms government less likely," Electoral Commissioner Tom Rogers said, referring to the additional time required to count postal votes.

With Australia going to the polls on May 21, rising living costs have dominated the final stretches of the campaign with voters rating it as the most critical issue in some polls.

Inflation has risen twice as fast as wages, keeping real income in the red and causing angst among voters as political parties awaited first-quarter wage data due later Wednesday.

Labor acknowledged the election would be close.

"We'll be working our butts off all the way up to the close of polls," shadow treasurer Jim Chalmers told ABC television.

  - Reuters