A community group fighting to prevent James Packer's six star casino being built on Sydney Harbour's foreshore alleges the planning body that signed off on it failed to fulfil its legal obligations.

Community group Millers Point Fund Inc is challenging a decision by the NSW Planning Assessment Commission to allow Mr Packer’s Crown to develop the casino on the waterfront at Barangaroo.

The foreshore site was originally designated as a public park with the casino to be built further back, but in June this year the commission approved the construction of the casino on the waterfront.

The Millers Point group has long voiced its concern about the space becoming an exclusive area, rather than an open space for all.

Millers Point member John McInerney said his group wasn’t completely against the Crown development, they just wanted it moved back closer to the rest of the city.

“It’s in the wrong position, it should be relocated and Mr Packer could have done that a year ago,” Mr McInerney said outside the NSW Land and Environment Court on Tuesday.

Environment Defenders Office, which is representing the Millers Point community group, admitted they faced a David and Goliath situation up against construction giant Lendlease, Crown and the Barangaroo Delivery Authority.

“Like all cases, there’s a winner and a loser and we think we are going to win,” NSW EDO chief executive Sue Higginson told reporters.

The Millers Point group says the planning body had wrongly thought the NSW parliament had decided on the site and that it was therefore legally unable to consider any alternatives.

Because of this misunderstanding, the planning commission did not assess the environmental suitability of the site, the group’s lawyer Michael Hall SC told the court.

Under the Environment Planning and Assessment Act, it is “mandatory” to assess the suitability of the site, Mr Hall says.

In 2012, the NSW parliament passed an amendment to the Casino Control Act that enabled a licence to be granted for the Barangaroo south area currently under scrutiny, the court heard.

Despite being sympathetic to community concerns about the development, the planning commission said parliament had decided on the location for the casino so its hands were tied, Mr Hall said.