Send them the bill for the rest of the fence repair and, when they refuse to pay, either take them to the small claims court or start proceedings at Fair Trading, then NCAT under section 233 (below). Tell them you will also be claiming all costs if they force you to do that.
This is just laziness on their part and there’s no reason you and your neighbours should have to pay because they can’t be bothered to claim.
233 Order for settlement of dispute between strata schemes
(1) The Tribunal may, on application by an owners corporation for a strata scheme, make an order to settle a dispute between that strata scheme and another strata scheme if:
(a) the strata schemes are contiguous or the dispute relates to a lease of land, or other arrangement relating to property, of one of the schemes, and
(b) the matter in dispute is not regulated by or under any other Act.
(2) In this section, a strata scheme is contiguous with another strata scheme even if it is divided by, or separated from the other scheme by, a natural feature (such as a watercourse), a railway, a road, a public reserve or a drainage reserve.