Alyson Chandler is organising the Palmerston North campaign. She says 71 per cent of those 40 million people are women and children.
Chandler says modern slavery is the severe exploitation of other people for commercial or personal gain. It is a human rights violation with exploitation at its heart.
It can take many forms - forced labour, human trafficking, debt bondage or sexual exploitation.
New Zealand has no accountability legislation on exploitation in the supply chain while some countries are looking to strengthen their legislation, Chandler says.
Without transparency in the supply chain, businesses may be unwittingly buying products with an element of modern slavery in the production.
A modern slavery act would make it easy for consumers to expect slavery-free products and services, she says. Currently, New Zealand businesses don't have an obligation to understand the risks of modern slavery.
Chandler says the campaign aligns with Trade Aid's values of using fair trade to help the marginalised and economically disadvantaged.
"It's an important issue, it really speaks to the heart of what we believe in."
People need to be respected and valued and have a sustainable living, she says.
On March 16, an open letter was presented to the Government by 85 New Zealand businesses expressing their support for progressing the work required for a modern slavery act. The signatories included Bunnings, Kathmandu, Nestle and Noel Leeming. Since then, other businesses have added their voices.
The petition closes on May 17.