Conservationist Tim Flannery and former TV host and Warringah candidate James Mathison were among the crowd as Ms Steggall said Sydney's northern beaches need a voice from "the sensible centre".
"He is a formidable opponent and I have never gone into a race thinking it would be an easy challenge," she told reporters on Sunday.
"I am prepared, I am supported and I know the views of the community."
Ms Steggall described Warringah as socially progressive and caring and pointed to the electorate's overwhelming support for same-sex marriage as an example of the local MP not representing his voters' views.
She said she supported economic stability and tax cuts from small and medium businesses and was opposed to changing negative gearing, franking credits and capital gains tax.
Like Kerryn Phelps in Wentworth, Ms Steggall will make climate change policy a key issue.
The former prime minister dismissed further action on climate change, saying locals cared about a northern beaches tunnel, lowered living costs, border security and power prices.
"Sure, we all want to do the right thing by the environment, but let's never forget that we're about 1.3 per cent of global emissions," Mr Abbott told the ABC on Sunday.
"It would close our economy down tomorrow and it wouldn't make a scrap of difference.
"And the last thing I want to do is to see our country impoverished for no good purpose."
Mr Mathison, a former host of Australian Idol, won 11 per cent of the Warringah vote in an anti-Abbott campaign at the 2016 election and says voter disillusionment with the long-time MP has only grown.
"I am pretty certain we'll see what happened in Wentworth happen here," he told AAP.
Liberal-turned-independent MP Julia Banks has offered her support to Ms Steggall, tweeting: "Australia and Warringah needs strong independent, genuine people like you".
Alice Thompson, a former adviser to Malcolm Turnbull, and indigenous activist Susan Moylan-Coombs have also announced they are running as independents in Warringah.
Mr Abbott faced nine opponents at the 2016 election, winning 51.6 per cent of first preferences and a margin of 11 per cent over Labor.
The seat has voted conservative since 1922.