Former Act president Catherine Isaac has been confirmed as No 2 on the party list.
Act will hope Ms Isaac, formerly known as Catherine Judd, will bring some much-needed credibility and stability to a party plagued by controversy and caucus in-fighting.
It is also weathering the fallout over the differences in opinion between leader Don Brash and Epsom candidate John Banks about decriminalising cannabis, and a poll on Sunday showing Mr Banks trailing National's Epsom candidate, Paul Goldsmith, by 14 percentage points.
A different poll last week indicated that support for Mr Banks would fall if National looked likely to win a majority of the party vote - enough to govern alone.
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If Act fails to gain 5 per cent of the party vote, it will need Mr Banks to win in Epsom to return to Parliament.
Ms Isaac was widely tipped to be the mystery candidate when the list was originally announced in August.
She declined to stand in previous years and was initially uncertain this time because of the health of her husband, Business Roundtable boss Roger Kerr, who is battling cancer.
Making public her candidacy will be seen as a sign she is ready for a full three-year term.
If Act wins Epsom and about 2.5 per cent of the party vote, she would join Dr Brash and Mr Banks in Parliament.
She is at present the managing director of a communications consultancy.