Under the approval, which comes with a number of conditions, Boral's subsidiary Dunmore Sand & Soil Pty Ltd will be able to extract around 1.35 million tonnes of sand from two new pits over three to four years.

It currently has permission to mine up to 800,000 tonnes of sand a year until 2030, and this rate of extraction will not change under the new approval.

The extra sand will help feed growing demand for sand in the Sydney construction market.

Read more: Locals prepare for stoush at Minnamurra sand mine hearing

Earlier this year, the NSW Department of Planning, Industry and Environment recommended the project be approved, saying its environmental effects were acceptable.

The Independent Planning Commission was then asked to determine the expansion after the department received objections from local councils and more than 50 members of the public.

Last year, almost 5000 people signed a petition objecting to the expansion, and several hundred people attended a protest against it.

In making their decision, commissioners Dianne Leeson and Peter Cochrane met with Boral, government officials, Shellharbour City and Kiama Municipal Councils and conducted an inspection of the site and surrounding area.

They also hosted an electronic public meeting to listen to the community's concerns at the end of October.

Issues raised included concerns about ecological impacts, ground and surface water impacts, acid sulfate soils, heritage impacts, Aboriginal cultural heritage, traffic and transport and impacts on local amenity and tourism.

The councils were concerned about clearing native vegetation, as well as the potential impact on the Minnamurra River and the wetlands to the east of the site.

The final decision was handed down on Monday (November 16), with the commission saying the "impacts associated with the Application are acceptable and in the public interest".

They said it would generate economic benefits for the region and NSW and would "provide ongoing social benefits through continuing employment".

"The application would extend the life of an established quarrying operation and would ensure the continued delivery of high-quality construction sand products to the Illawarra and Greater Sydney region," the commission said it it's statement of reasons.

"The continued development and recovery of a state significant sand resource is an orderly and economic development of the site."

"Impacts on water quality, groundwater, flooding, coastal wetlands and flora and fauna have been adequately assessed and where there are predicted impacts, mitigation measures have been implemented as far as practicable."

The commission also stated that things like air quality, noise and visual impacts could be mitigated in accordance with its imposed conditions, traffic and transport impacts could be managed subject to the preparation and implementation of a traffic management plan and impacts on historic heritage values, like Dunmore House, were relatively low and could be adequately managed by the imposed conditions.

"In relation to Aboriginal cultural heritage, the commission acknowledges that a massacre of Aboriginal people is likely to have occurred in the broader area and is unlikely to have occurred within the proposed disturbance area," the decision said.

"The commission finds that the current legislative framework provides appropriate protection against any further impacts to heritage values and has therefore imposed conditions of consent to ensure the preservation and protecting of Aboriginal objects and Aboriginal places within and also outside the site."