What remedies are available in the last instance?

Electoral Act 1968, 121(g) – (j)

if the High Court determines that the respondent was duly elected, suchelection shall be and remain as valid as if no petitionhad been presented against it;

(h)       if the High Court determines that therespondent was not duly elected, but some other personwas or is entitled to be declared duly elected the respondent shall forthwithbe deemed to have vacated his seat, and theHigh Court shall forthwith certify its determination to the Secretaryand the Secretary shall thereupon by notice in the Gazette declare such other person duly elected;

(I)                if the High Court determines that therespondent was not duly elected and that no otherperson was or is entitled to be declared duly elected the seat of therespondent shall be deemed to be vacant andthe High Court shall certify its determination to the President that avacancy has occurred, the cause of such vacancy and the constituency in whichsuch vacancy has occurred;

(i)        where, on the trial of an electionpetition, the High Court determines that the respondentwas not duly elected and is of opinion, having regard to the circumstances, that it would be just and reasonable to relieveany party to the action from all or a portion of the costs thereof, then‑

(i)if the High Court finds that the election of the respondent was dueto a mistake or improper performanceor failure of performance of any function bona fide made by any officer, itmay, after sufficient notice to the Director of Public Prosecutions to show cause to the contrary, make such order as to thepayment by the State of the costs of the action or portion thereof, asit may deem fit;

if the High Court finds that the election of therespondent was due to a mistake or improper performance or failure of performance ofany function mala fide made by any officer, it may, after sufficient notice tosuch officer to show cause to the contrary, make such order as to the paymentby such officer of the costs of the action or portion thereof, as it maydeem fit.

http://aceproject.org/regions-en/eisa/BW/Electoral%20Act%201968.pdf