The Lupane East seat fell vacant following the death of Zanu-PF MP Sithembile Gumbo on April 5.
The Zimbabwe Election Support Network, in a report on Monday, said they also observed the nomination court for the by election on 14 June.
Nine candidates will face off in the by election.
“Veritas has raised concerns that the president did not comply with the Constitution in proclaiming the date for the conduct of the by-election. The election will be held after the 90 days period within which a vacancy should be filled in accordance with both Sections 158(3) and 159 of the Constitution, and Section 39 of the Electoral Act,” said ZESN.
The organisation said some candidates faced challenges as they only discovered at the nomination court that they were required to pay US$50 or RTG$300 as nomination fees.
“Problems with the swiping machine further caused delay as the ZEC officers had to replace it. Some of the candidates were also struggling to fill in the required forms, repeatedly being called to correct mistakes. At the time the Court closed, applicants were granted 30 minutes to correct mistakes on their applications.
“ZESN is of the view that this challenge could have been easily averted had the aspiring candidates took time to familiarise with the forms that can be found in the Electoral Act ahead of the Nomination Court day,” said ZESN.
