The programme came about after the infamous Roast Busters scandal, in 2013, in which teenage boys were discovered bragging on Facebook about having sex with drunk and underaged girls.
Now a national group of sexuality and health education experts - made up of teachers, researchers, practitioners and sector leaders - have raised ongoing concerns about the move to extend the programme to all high schools.
"Spending more than $18m on this programme is an outrageous waste of public money.
"That money should go to support teachers and schools to deliver excellent programmes," a statement said.
Mates & Dates is taught at all year levels, years 9 to 13, by outside providers such as youth workers and counsellors.
Group spokeswoman, University of Auckland professor Dr Katie Fitzpatrick, said a more sustainable and long-term plan was required to address issues of sexual health and consent.
"We absolutely must invest in relationship, consent and sexuality education in every school and it needs to be delivered by teachers.
"It is irresponsible that such a significant sum of money is being used to fund this programme when it is being taught in a way that is inconsistent with effective education practice and education policy.
"This is funding that will not go to teachers or to schools. Instead, it will fund only five lessons for students, delivered - at a huge cost - by outside providers."
The group acknowledged that a number of schools had reported positive results with the Mates & Dates programme.
However, it cited other feedback from health education teachers and Māori-medium schools that said the programme was not meeting the needs of their students.
The one-size-fits-all approach was also flawed, they said, particularly when it came to teaching the topic to Māori and Pasifika students.
"The most recent evaluation of Mates & Dates [ACC, 2017] indicated that 36 per cent of students did not rate the course as good, with the evaluation also stating that boys and Pasifika students were most likely to be underserved by the programme.
"The decision to expand the investment in Mates & Dates by such a large sum has come as a surprise to the health education sector."
The group also said stakeholders and experts in the area were not consulted in neither the development of the teaching resources provided in the programme nor the teaching approaches used.
