Critics including academics, the union, opposition MPs and teachers say they believe the erosion in quality is partially linked to the rise in profit-driven childcare, which means less money for quality teaching and provision.
However ERO figures showed that while kindergarten, which is community based with 100 per cent qualified teachers, gets better reviews, there was very little difference between community-based educare and private educare centres.
In calling for an inquiry, National President Louise Green said there was no evidence that market forces provide quality education in the compulsory public education sector.
"So why has the government allowed the market to become so prevalent in the early childhood sector?" she said.
"There appears to be no Ministerial interest or responsibility being taken in the provision of quality early childhood education. The government has simply said that the market will sort it."
"We need an inquiry into why the government believes market forces will work in the early childhood sector when they patently don't work in the compulsory sector.
"Early childhood education is far too important to be left to market forces."
Reaction from the minister and ministry is being sought.
When approached about corporate ECE yesterday, the ministry said privately-run ECE services have always played an important role in early childhood provision in New Zealand.
"We have not raised concerns about any potential increase in the number of privately-run providers. In fact, we believe they play an important role in widening provision of early childhood education in areas that are currently underserved."
If an inquiry was held, it would follow a taskforce investigation from 2011 and sector advice from 2012 that is yet to be wholly implemented. Those inquiries did not address the differences between for-profit and not-for-profit care.