Picture walking on an aerial walkway that reaches the tree canopies and seeing orang utans up in their natural environment.

That is what visitors to the new Rainforest Park in leafy Mandai will get when the park opens in 2021.

As visitors wander through the South-east Asian forest, they will see the animals in a seemingly cage-less environment.

Other than the aerial walkways, where visitors can interact with arboreal primates, the park will have boardwalks on the ground level and an underground cavern with geological formations.

Said Mr Mike Barclay, chief executive of Mandai Park Holdings: "The philosophy we want to have is that visitors will be in this environment with the animals... and get really close to them."

While the animals may appear close enough to touch, there will be measures to ensure the safety of both humans and wildlife. These include keeping the aerial walkways far enough from the trees where the primates live.

There will also be trekking and adventure activities at the new park.

Mandai Park Holdings provided details of the park yesterday during a ceremony to celebrate the start of work on the hub of five wildlife parks it is building in Mandai.

Besides the Rainforest Park, the hub will include the Bird Park, which will be moved from its current premises in Jurong, Singapore Zoo, Night Safari and River Safari. The concept of a cage-less zoo for the Rainforest Park was first mentioned in January 2015.

Mr Barclay said: "There are 5,000 South-east Asian species which are facing extinction in the wild, and we want to tell some of those stories and encourage people to build an affinity with these species, as we also do work in the field to try and protect them."

When the new Bird Park is opened in 2020, visitors will have access to large aviaries with a variety of birds flying freely.

The hub - inclusive of accommodation options - is expected to fully commence operations by 2023.

Mr S. Iswaran, Minister for Trade and Industry (Industry), who was at yesterday's event, said: "With a strong focus on conservation and sustainability, Mandai will be a hub for nature education and research, as well as a leading nature destination in Asia. Visitors will be able to retreat from the bustle of city life, be reinvigorated by nature, learn about animal conservation, and spend quality time with family and friends while cycling or hiking."

Wildlife consultant Subaraj Rajathurai welcomed the South-east Asian focus of the new Rainforest Park. He said: "It educates the public on the value of South-east Asian wildlife and, more importantly, on the perils and vulnerability they are in, including the ones in Singapore."

Primate researcher Andie Ang is concerned about the mix of native and non-native primates as the Rainforest Park sits so close to the Central Catchment Nature Reserve. This is especially so as the critically endangered Raffles' banded langurs have been sighted in the area.

Dr Ang said: "The mixing of native and non-native primates within a confined space could result in interbreeding or competition, both of which could cause a decline in native populations."