About 500,000 extra people will go to emergency departments to avoid a GP co-payment, with an added cost of $80 million, NSW opposition claims.

Emergency departments in NSW will be flooded with an extra half a million people a year if the federal government introduces its GP co-payment, internal health department documents reveal.

The move would increase emergency department costs by $80 million a year according to a preliminary study prepared for the NSW government in May.

The analysis by NSW Health was based on a $6 GP fee, rather than currently planned $7 fee.

NSW Opposition Leader John Robertson says the co-payment would be a "disaster".

"It will smash the NSW health system," he said in a statement on Wednesday.

"Thousands of people will be forced to turn up in emergency departments to avoid paying the fee to their local GP."

The documents were obtained through a call for papers by the NSW Legislative Council on the impact of the GP co-payment on NSW.

There were 2.6 million presentations to NSW emergency departments in 2012-13.

However, the study shows that figure would jump by 27 per cent with 500,000 extra attendances.

"The GP tax will be a disaster for families using emergency departments," Mr Robertson said.

NSW health minister Jillian Skinner said the study used "rudimentary scenarios" resulting in "preliminary" figures.

"[NSW Health] has undertaken no detailed modelling on potential impacts since the federal budget handed down in May, and I have not commissioned any modelling," she said.

Copyright AAP 2014.