Gretel Killeen has reflected on the incredible success of Big Brother 25 years after it first hit Australian screens – and revealed she almost didn't audition because she believed she was already 'too old' for TV.

The veteran presenter, 63, tells this week's Stellar magazine that she was convinced she had already aged out of the industry's beauty standards when she landed the groundbreaking hosting role at just 38.

Looking back on the career-defining opportunity, the Sydney-born presenter said she couldn't imagine herself being chosen to front a prime-time reality TV show.

'When I was asked if I wanted to audition for it, I remember thinking: I'm 38, I've got dark hair, and I'm a woman – there's no way in the world they're going to want me,' she began.

'I thought 38 was too old and you had to have blonde hair to be a host. Those were the parameters in those days.'

Instead, Gretel went on to become one of Australia's most recognisable television personalities, fronting Big Brother during its cultural peak and breaking new ground as one of the country's first female hosts of a major live reality show.

Reflecting on the era, she said the show's impact is difficult to explain to younger audiences, who have grown up in a completely different world.

'To put that in perspective, it's kind of unimaginable now... we didn't have YouTube. Facebook didn't exist,' she recalled.

'Remember in those days, if you wanted to text, you had to press A four times to get D... all of that business.'

'It was the first reality show, but there were other firsts. To have a female hosting, that was unheard of. Not as a sidekick, but hosting a live show.'

Gretel also reflected on how the industry viewed women when she first began her career.

'When I was first doing voiceover work, there used to be a theory that men got eight times more work than women. This was in my early 20s,' she said.

'We got the same money per hour, but men got eight times as much work. This was the theory, because men didn't want to hear women's voices on the radio or on television, because it reminded them of their wives or mothers nagging.'

She credited the men who backed her for helping change Australian television.

'There were men supporting me, who not only did that, but they weren't hovering,' she said.

Now preparing to return to television as the host of The Traitors Australia, Gretel said her perspective on age has changed dramatically.

'I didn't think I was young at the time [at 38] but in retrospect, I was!' she admitted.

'So this is karma. I'm a lot older... by older, I mean a lot of experiences have occurred.'

The presenter also reflected on the overwhelming fame that came with Big Brother, admitting it changed every aspect of her life.

'The fame I experienced with the first Big Brother was so enormous... so unrelatable nowadays, because there are so many people in the spotlight,' she said. 'I was a young, single mum.'

These days, however, Gretel says she's far more comfortable in her own skin and no longer worries about trying to fit the mould she once believed television demanded.

'I know more who I am,' she said.

Looking ahead, she said she has no plans to slow down.

'A lot of people at literary festivals are women. There are women's trekking groups, women's art groups. A lot more women volunteer than men. This is where we have a choice: I can either call myself old and slow down, or sprint to the finish line.'

Gretel originally hosted Big Brother on Ten from 2001 to 2007, before being replaced by Kyle Sandilands and Jackie 'O' Henderson.