Saturday afternoon's March For Nature in Auckland will see protesters march from Aotea Square down Queen St.
The organisers said they were expecting thousands of people to turn out for the protest against the fast-track legislation and what they called the coalition's "general war on nature".
Auckland Transport is warning city centre visitors of road closures and traffic between 12pm and 4pm.
"There will be some detours to city centre bus routes," it said, asking people to plan ahead.
AT said the city centre would still be accessible by public transport.
But access to the Downtown Car Park may be restricted or closed for a short period of time, it said.
Other March for Nature protests would be held around the country, including in Nelson, Whakatāne, Christchurch and Tākaka.
Meanwhile, in Wellington, the People Over Profits march will protest government cuts to public services and job losses.
As of this week, about 6000 jobs had been cut in the public sector as the government looked to make $1.5 billion in savings.
Police said they were aware of the protests and advised anyone travelling in the affected areas to allow time for their journey.
