Sally McManus, 45, is the front-runner to replace Dave Oliver as secretary of the Australian Council of Trade Unions after his shock resignation on Tuesday.

His successor will be decided by the left-wing dominated ACTU executive within the coming weeks and is now being debated behind closed doors.

As ACTU vice-president and head of campaigns, Ms McManus is credited for mobilising an "army" of volunteers to doorknock and call voters in the lead-up to last year's federal election.

Her achievements as a left-wing secretary of the NSW and ACT branch of the Australian Services Union included a successful campaign for equal pay.

Natalie Lang, who replaced her as the head of the ASU in NSW and worked with her for more than a decade described Ms McManus as "the most impressive union campaigner I have ever had the pleasure of working with".

"She has always said there is no silver bullet to a campaign," Ms Lang said.

"Social media alone is never going to win a campaign. We have to be able to push every button and pull every lever, but turn out hundreds of thousands of workers. And that's what she's always been able to deliver across many industries."

Ms Lang, who started working for the ASU as an organiser, was deputy secretary to Ms McManus before she left to work for the ACTU.

"It is five years to the day since we won equal pay for the community sector," Ms Lang said on Wednesday.

"Sally McManus led that campaign nationally. She threw everything and more at that campaign."

About 5000 union members danced outside the officer of former NSW Premier Barry O'Farrell to the Elvis Presley song  A little less conversation, a little more action during that long campaign.

The equal pay case was fought on behalf of 150,000 social and community service workers across Australia. Wage increases of between 23 to 45 per cent were won in 2012 for the female-dominated industry.

"These are historically huge wage increases to win," Ms Lang said.

"The equal pay case was run on the basis of the gender under-valuing of care work."

Sydney radio presenter Tim Brunero remembers being impressed and in awe of Ms McManus when she was a senior student at Carlingford High School and he was starting Year 7.

The pair formed an unlikely friendship through their shared interest in sailing.

"I was a keen sailor and the McManus family were keen sailors. I remember going to sailing tournaments and she was very supportive and lovely when I was only a little pipsqueak 12-year-old," he said.

"I think she was sophisticated and intelligent and ... was not talking down to me or patronising me.

"She wanted to hear what I had to say and debate things with me. That was very new to my experience."

Union movement observers say Ms McManus has been a "poster child" for a new breed of unionist with cutting-edge social media skills.

But some senior Labor Party and union figures privately say they are not yet convinced that Ms McManus is the right person for the top job.

Former ACTU assistant secretary Tim Lyons unsuccessfully contested the ACTU secretary's job in 2015 when he criticised the leadership as ineffective and out of touch with members.

Nadine Flood, national secretary for the Community and Public Sector Union the sent a message to rank and file leaders and staff on Wednesday night ruling out her contention for the secretary job.

"While I appreciate people who've thought to mention my name, I am not putting my hand up for the ACTU Secretary position, she said.

"Our movement has great people to take on senior leadership roles, including two dynamite women leaders in Ged Kearney and Sally McManus at the ACTU."

Ged Kearney is the third female president of the ACTU, following Sharan Burrow and Jenny George, who was appointed as the first in 1996. The president position sits below the secretary in the ACTU hierarchy.

As a woman, Ms McManus represents the new face of the union movement in Australia.

The ACTU gender audit report last year showed that 54 per cent of the total union membership in Australia is made up of women.

Rae Cooper, Associate Professor in Work and Organisational Studies at the University of Sydney Business School, says the typical union member is no longer a construction worker in a hard hat. She says women are more highly unionised than their male colleagues for the first time since national union data was collected.

Union membership for men has fallen from 43 per cent to 14 per cent. For women, it has fallen from 35 per cent to to 16 per cent. The highly unionised teaching and nursing professions are dominated by women.

"Women are now well and truly in the mainstream, not at the margins, of union membership," Dr Cooper said.

"The challenge for unions, in the context of a significantly gendered reconfiguration of union membership, is to make themselves more relevant to their new heartland."

Dr Cooper's research suggests there is a mismatch between the changes in membership and the leadership, culture and "business" of unions.

"There is a clear gendered hierarchy, not dissimilar to that seen in organisations in the private and public sector within unions," she said.

"It makes perfect sense and there is a clear business case for the leadership of the union movement reflecting the feminised membership in exactly the same way that it makes sense in government, not for profit and in business leadership roles.

"It is about time that women took their place in these most senior positions across all sectors and there is almost no clearer case than in the union movement. They also need to make their agendas and campaigns fit better with the realities of working women's lives "

Do you know more? apatty@fairfaxmedia.com.au