A "secret deal" between the NSW government and port owners is costing businesses and consumers thousands of dollars, according to the state opposition.
The opposition claims the coalition's "scandalous" deal effectively stops Newcastle Port - which was privatised in 2013 - from developing a container terminal.
The consumer watchdog in April said it was investigating whether the privatisation of the Port Botany and Port Kembla leases in 2012 was potentially anti-competitive.
Under the terms, Newcastle is obliged to pay NSW Ports - which owns the Botany and Kembla leases - for loss of container business if it handles more than 30,000 shipping containers a year.
Labor on Wednesday said this was resulting in higher prices for consumers.
"Minister Melinda Pavey has to explain why her government is imposing high transport costs on businesses that import and export from northern NSW," opposition roads and freight spokeswoman Jodi McKay said in a statement.
Industry sources have told the opposition it costs about $1500 for a truck to make a return trip from Port Botany to Newcastle.
"The easy way to cut transport costs and help make businesses more competitive would be to let them move their products through Newcastle - instead of travelling a further 180 kilometres south to Port Botany," Ms McKay said.
The ACCC investigation is expected to be completed by the end of 2018, its chairman Rod Sims told AAP in April.
At the time, Premier Gladys Berejiklian said she wasn't worried about the probe because the watchdog "was consulted during that (privatisation) process".
Ms Pavey has been contacted for comment.