The New South Wales supreme court has set aside a Minns government decision to cut off access to Newcastle harbour to try to prevent a four-day climate protest.

The court found the notice was invalid after hearing an urgent application from climate activist organisation Rising Tide on Thursday.

Rising Tide argued the four-day marine exclusion zone that was to commence at 5pm on Thursday was an improper use of a section of the Marine Safety Act, which was intended to ensure special events proceed safely – not to prevent them from taking place.

Justice Sarah McNaughton accepted these arguments, telling the court that although lawyers acting on behalf of the transport minister, Jo Haylen, made “skilful submissions”, the wording of the relevant section was not designed to “prohibit the special event it purports to be regulating”.

McNaughton noted that while she had quashed the notice for the exclusion zone – which could have meant on-the-spot fines of up to $1,100 for anyone who breached it – this did not mean certain activities by Rising Tide protesters would be lawful.

This is because an earlier supreme court decision allowed police to deny the protesters’ form 1 application, meaning they are not protected from being charged under obstruction and unlawful assembly offences if they block the waterway.