Voters began casting ballots in a Sydney by-election that could determine the future of Australian Prime Minister Malcolm Turnbull, who is hoping to regain his parliamentary majority and safeguard his leadership.

The election campaign in the blue-ribbon Sydney seat of Bennelong has been bitterly fought between the centre-right government’s candidate, former tennis star John Alexander, and the labor opposition’s Kristina Keneally, a high-profile former premier of New South Wales, Australia’s most populous state.

The outcome of the election could have serious repercussion’s for Turnbull’s conservative coalition.

If Alexander loses, the government will again be forced to govern in minority and rely on the support of independent lawmakers.

Results of the by-election may not be announced until Sunday, due to the high number of postal votes made before the election and which are always counted last.