LEIGH SALES, PRESENTER: The One Nation preferences I just asked Angus Taylor about could be all important in the upcoming federal election, especially in marginal seats.

But what exactly happens with preferences and how much influence do political parties have over where they go?

The ABC's election analyst, Antony Green, explains.

SCOTT MORRISON, PRIME MINISTER: See I'm not interested in getting One Nation's preferences. I'm interested in getting their primary vote.

ANTONY GREEN, ABC'S ELECTION ANALYST: For all the talk of preference deals, the reality is that parties cannot direct preferences.

The only preferences that count are the numbers written on ballot papers by voters themselves.

All that parties can do is try to influence what voters write either by distributing how-to-vote material or by sending subtle messages to minor party voters about what they should do with their preferences.

Scott Morrison has been clear that what he wants voters to do is give the Coalition their first preference but he also knows that since One Nation formed two decades ago, its greatest success has always come at the expense of the Coalition.

The Coalition's concern is to maximise the flow of preferences back from One Nation voters.

Labor has had the same problem in losing voters to the Greens but almost every vote lost to the Greens comes back to Labor as a preference. Green voters are more aware of which side of politics they inhabit.

The One Nation voters are much less disciplined.

At the 2016 federal election only half of Queensland's One Nation preferences flowed back to the Coalition.

It was the drift of One Nation preferences to Labor that defeated former Liberal MP, Wyatt Roy, in his Queensland seat of Longman and delivered Labor a narrow victory in the Townsville seat of Herbert.

The most recent state elections in Queensland and Western Australia have shown that the Coalition can get stronger flows of preferences if they put One Nation high on their how-to-votes and One Nation reciprocates.

But in both elections the controversy created about those how-to-votes nullified the advantage gained by the extra preferences.

For Scott Morrison, attacking One Nation directly risks alienating a potential source of preferences but getting too close to One Nation risks alienating moderate voters.

And then there is the geographical balance. In North Queensland where One Nation polled well, you need a different message to Sydney and Melbourne where you could easily alienate swinging voters.

It's a difficult balancing act for the Prime Minister.