Voting for the March 23 state election opened at 8am on Monday, with some 1.5 million people expected to vote before election day.

Australian Political Studies Association vice president Rodney Smith expects early voting will increase to more than 30 per cent - up from 25.1 per cent at the 2015 election.

Early voting was just 15.3 per cent in 2011.

"Most of the increase will be in early in-person voting, with a small increase also likely in the iVote," Professor Smith told AAP.

"The main causes in NSW as elsewhere are socio-economic changes which make Saturday voting at a local polling place less convenient than it once was.

"In response, legislators have made it progressively easier to cast an alternative vote, including introducing the iVote in NSW."

iVote was used by about six per cent of voters in 2015.

It's available to silent electors, those outside NSW on election day and those with other difficulties voting in person.

Voters need to register either online at ivote.nsw.gov.au or via phone at 1300 2 iVote to participate.

About 14.2 per cent of voters in 2015 pre-polled in person while about five per cent voted via the post, according to NSW Electoral Commission data.

Australian Associated Press