The NSW Electoral Commission received 640,291 prepoll votes and 284,000 internet votes (iVotes) before the March 28 state election this year.

Dr Peter Chen, a senior lecturer at the University of Sydney, estimated that the proportion of early votes for the 2015 election would be in the order of about 25 per cent after postal votes are counted. More than five million people were enrolled to vote this year.

Figures from the electoral commission show that 15.3 per cent of the 4.3 million people who voted in the 2011 NSW election, did so before polling day.

Dr Chen said election campaigns traditionally ran a narrative over a full election campaign and major policies were traditionally announced late in that cycle.

"Once upon a time a campaign used to be a highly structured event over four weeks," he said. "That assumed you control the narrative through mass media to a single destination date on polling day."

But with the increasing trend in people voting early, political parties were putting out more frequent "bite-sized" policy announcements earlier and more frequently throughout the campaign.

"Parties can no longer assume they have time to construct a broader narrative over the campaign," he said.

"This changes the political process."

Dr Chen said parties were door knocking with postal votes in hand, encouraging people who support them to vote early. This would help ensure the voter had less time to change their mind.

Busy white-collar workers who worked long hours were among those more likely to vote before election day. These people were more likely to be directly targeted by political parties.

Because parties were focused on "winning each day" instead of developing a narrative over time, "a proportion of the electorate will be deciding the vote on less information than before".

Dr Chen said voters tended to cut government's more slack if they were presented with a broader vision of their policy direction.

Greater urgency in getting political messages out had resulted in the impression that this year's NSW election boiled down to two issues - privatisation and coal seam gas mining.

Labor strategist Bruce Hawker said it was now imperative for parties to get their major messages out early, in advance of pre-poll deadlines.

"Generally we'd leave some of our major commitments to the last week to give a sense of momentum," he said. "We used to focus the great bulk of our advertising [then] too."

A Liberal party strategist said the focus of campaigns were still on mainstream voters who mostly voted on Saturday.

"We need to do a lot more work before prepoll. But to me pitching the campaign around early voters specifically doesn't make sense because they are not the majority.

"Early voting is no different a snapshot of what is happening in the campaign than what happens on polling day."

Voters must notionally provide a reason for casting their ballot early. Accepted reasons include caring for someone who's sick, prohibitive religious beliefs and being in prison.

Voters need only verbally state their reasons for voting early when showing up to their local polling booth. Those voting outside a local district must do so in writing.