The byelection for Wentworth, Malcolm Turnbull’s former seat, will be held on October 20.

The announcement by the Speaker, Tony Smith, came ahead of the Liberals choosing their candidate on Thursday. Rolls will close on September 24 and nominations will close September 27.

The byelection is crucial for Scott Morrison who will face a very difficult test in the initial days of his prime ministership. The outcome in the byelection will set the political tone for the rest of the year.

Morrison will encounter a lot of anger in the electorate at the removal of Turnbull, who was very popular and had grown the Liberal vote substantially, to a 17.7% margin in 2016. On what is known of his arrangements, Turnbull would be back from New York in the final days of the byelection - whether he would campaign for the Liberal candidate is not known.

Read more: View from The Hill: Wentworth looks scary for Liberals having trouble explaining why they sacked its PM

Labor’s candidate, Tim Murray, has already been campaigning, and high profile Kerryn Phelps, who has a medical practice in the electorate, is set to run. Phelps could attract a strong vote from disaffected Liberal supporters.

In the Liberal preselection all eyes will be on whether a woman is selected, after one of the frontrunners, Andrew Bragg, pulled out, saying the candidate should be female.

Read more: Bragg drops out of Liberal preselection battle, calling for woman candidate

The two leading women in the field are Katherine O'Regan, who has been a political staffer and is chair at Sydney East Business Chamber, and Mary-Lou Jarvis, a vice-president of the NSW Liberal party.

Earlier this week Morrison, asked on 2GB – in the context of the push for a female candidate - whether he was “a merit person or not”, said “I’m a merit person and the party members will decide our candidate in Wentworth.”

“It’s [the preselectors’] choice … Just like it has to be, in every single seat in the country,” he said.

Michelle Grattan does not work for, consult, own shares in or receive funding from any company or organisation that would benefit from this article, and has disclosed no relevant affiliations beyond their academic appointment.