About 19,000 votes cast for the NSW upper house will be considered valid, despite two parties being left off part of the ballot paper, electoral authorities say.

Both parties, the Outdoor Recreation Party and the Animal Justice Party say they will challenge the election result in court if they narrowly miss out on an upper house spot.

The state's electronic voting system iVote was suspended for almost five hours on Tuesday after it emerged the two parties were omitted from the "above the line" section of the ballot paper.

The NSW election will be held on Saturday March 28. But voters who cannot make it to a ballot box in person – such as those who are blind, live more than 20 kilometres from a polling booth or will be away from NSW on election day – can use iVote to vote online or over the phone.

However on Tuesday it emerged the Outdoor Recreation Party and the Animal Justice Party had been omitted from the "above the line" section of the online ballot paper, due to human error.

Senator David Leyonhjelm, the Outdoor Recreation Party's registered officer, said the error may have disadvantaged his party and the votes should considered invalid. He indicated that should his party be narrowly defeated, it would lauch a legal challenge to the election result.

The Animal Justice Party registered officer Mark Pearson, said: "If we are on a knife-edge in the Legislative Council and its clear we haven't got in because of a handful of votes, we will have to seek remedy in the Supreme Court in relation to those 19,000 votes."

Votes cast for a group or party above the line are considered a vote for each candidate of that group in the order listed below the line.

To vote below the line electors must number at least 15 candidates in order of preference.

A NSW Electoral Commission spokesman said while the parties were left off the above the line section, the names of the respective candidates for these groups appeared below the line.

About 19,000 people had cast an iVote before the system was suspended, and their votes would be "considered valid", the spokesman said.

"People who wished to vote for the two groups concerned were able to simply vote for the group's candidates below the line," he said.

"All votes were correctly recorded as per the voter's intention."

The spokesman said those who submit an iVote can verify it afterwards, and change their vote at any point until 6pm on election day.