In a statement on Monday afternoon, the party said it was in mourning for Christensen, who was also New Zealand's only registered specialist poultry veterinarian.
"I wish to pay tribute to Neil, who was an infectiously charismatic and fascinating man," leader David Seymour said in the statement.
"He was a dedicated member of the ACT Party who will be immensely missed.
"We ask that media respects his family's request for privacy and allows them to mourn in peace."
Christensen's death will trigger a by-election for the Port Waikato electorate.
Party votes will be counted in the General Election results. Port Waikato electorate votes will not be counted, and a by-election will be held after the election to elect a member of Parliament.
When the preliminary results are announced on election night, and the official results are released on 3 November, they will be based on a 120-seat Parliament with 71 electorate MPs and 49 list MPs (unless there is an overhang). When the by-election is held in Port Waikato and an electorate MP is elected, it will increase the size of Parliament to 121 seats.
A date for a by-election will be set by the prime minister.
The Electoral Commission extended its condolences to the family of Christensen.
Chief electoral officer Karl Le Quesne said the thoughts of commission staff were with his family.
"It is an unusual situation for a candidate to pass away before election day, and the Electoral Act sets out what happens next.
"For voters in the Port Waikato general electorate - if you haven't voted yet, please go and vote. If you have already voted, you don't need to do anything more," said Le Quesne.
"Your party vote will be counted and contribute to the overall results of the General Election.
"Your vote is important. You will have an opportunity to vote for a member of Parliament for the Port Waikato electorate when a by-election is held."
