"It's not about asking for money, so that we can sit on the couch and eat Tim Tams and watch Oprah re-runs.
"It's about asking for support, because when you're a new mum it takes longer than 12 weeks to work out who you are as a mum, who your baby is and reconfigure your family and just get into the biggest role of your life," she said.
The bill, which passed its first reading in parliament at the end of July, is opposed by the National and Act parties.
Finance Minister Bill English said in April extending parental leave to 14 weeks would require an extra $500 million in borrowing over the next three to four years.
But Department of Labour figures show that in three years, it would only cost $315.6 million each year, $145.7 million more than the forecast $170 million cost of keeping it at 14 weeks.
Ms Brown, who returned to work 14 weeks after the birth of her son, said many parents needed longer than the assigned 12 weeks.
"It would be wonderful to support mothers and fathers during this really critical time...and not rush them back into work.
"Often their sleep deprived, the babies are not sleeping through the night and it can lead to post-natal depression," she said.
"Let's support our mums, let's support our families so we can put a wonderful generation of New Zealanders into the world that feel secure."
Mob participant Tamara Croft said many parents needed more assistance after a baby was born.
"We lag pretty much the entire OECD in parental benefits for young children.
"I've got a niece and two nephews who my brother is raising by himself. He had to leave work to be able to look after them...[but] it's one of those situations where you see it's really important for parents to spend time with their young children," she said.
After their Britomart boogie, the flash mob packed into two special carriages reserved for them on a Newmarket-bound train.
Campaign organiser Georgia Bates said submissions to on the bill would be filled out at the Newmarket station.
