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.
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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.