Illawarra Coke Company (ICC) director Kate Strahorn anounced the company's decision in Wollongong on Wednesday, saying ICC would undertake to protect and manage the ecological communities present in the area concerned.

ICC wants to develop part of the 118ha site for housing, following its development of the old cokeworks site in Corrimal.

The company will earn biodiversity credits from the government for the stewardship agreement, and may be able to use the conservation areas to offset damage to other areas by development. 

The credits from stewardship agreements can also be traded or sold.

"We understand the importance of the ecological communities on the former Coalcliff cokeworks property and are pleased to have signed this agreement," Ms Strahorn said.

Ms Strahorn said it was too early to say whether the biodiversity credits would be used to help smooth the way for housing development.

But while ICC announced it had finalised the agreement, the details of the agreement itself were not on the NSW Office of Environment and Heritage website on Wednesday.

Ms Strahorn said the agreements were "commercial in confidence". But some other similar agreements are available in full detail to the public on the website.

The area ICC said was subject to the agreement is mostly contained to the north of the cokeworks site, northwest of Coalcliff train station. A large section to the south of the site is also included. 

Neither of these preservation areas include the prime land for housing development, which is a flat clear area to the east of the property.

Ms Strahorn said no decisions had been made on the development of the site.

"The former Coalcliff cokeworks property is complex in nature and we are working through a comprehensive list of studies to better understand the best uses for this portion of the property," she said.

Part of the land may be used for public recreation, Ms Strahorn said.