Deputy Prime Minister Barnaby Joyce could face a Melbourne Cup-sized field of candidates in a New England byelection, with Labor, One Nation, the Shooters, Fishers and Farmers Party, and former independent MP Tony Windsor all looking to stand.

The Nationals are already making preparations in case the High Court orders a byelection, with campaign booth 'captains' contacted and rosters being prepared for volunteers to work on pre-poll stations.

The High Court this week held three days of hearings on whether the so-called "citizenship seven" - Mr Joyce and six senators - were validly elected in accordance with section 44 of the constitution, which prevents dual nationals from serving in Parliament. Mr Joyce discovered in August he was a New Zealand citizen by descent.

Chief justice Susan Kiefel has said the bench will endeavour to deliver rulings "as soon as possible".