Is there a legally mandated filing fee and/or deposit?

Electoral Act 1968, 138

Cost of petition

All costs, chargesand expenses and incidentals to the presentation of an election petition and to theproceedings consequent thereon shall, subject to paragraph (j) of section 121 bedefrayed by the parties to the petition in such manner and in such proportionsas the High Court may determine, regard being had to thedisallowance of any costs, charges or expenses which may, in the opinion of theCourt, have been caused by vexatious conduct, unfoundedallegations or unfounded objections on the part either of the petitioner or therespondent, and regard being had also to the discouragement of anyneedless expense by throwing the burden ofdefraying the same on the parties by whom it has been caused, whether suchparties are or are not on the whole successful.

(2) Costs may be taxed and recovered in the same manner as the costsof civil proceedings in the High Court.

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