Two large corporates - Genesis Energy and DB Group - had made Simplicity their preferred KiwiSaver supplier, while talks were underway with a number of other organisations, Stubbs said.
Simplicity will use US low-cost fund manager Vanguard which employs a passive investment approach.
Passive funds track market indices and cost less than active funds where stock trading requires more work.
Based in Pennsylvania, Vanguard manages approximately US$3.4 trillion in assets and has thrived on undercutting fees in the US and around the world.
Fees for the average KiwiSaver are currently 1.3 per cent a year.
Stubbs, who is self-funding the start-up, has said Simplicity's administration fee will be just $30 a year, plus a 0.30 per cent a year fund management fee.
More than 2.6 million people are signed up to KiwiSaver with around $34 billion invested.