A nationally significant koala population could be wiped out within a couple of decades after the federal government approved an upgrade of the Pacific Highway that bisects a key colony, environment groups say.

Josh Frydenberg​, the new Environment and Energy Minister, approved the four-lane expressway's new route near Ballina on the NSW North Coast on July 19, the day he was sworn in, the groups say. The NSW Roads and Maritime Services made the approval public only on Monday – a month later.

"We were disappointed by the lack of transparency," said Josey Sharrad​, campaign manager for the International Fund for Animal Welfare.

The RMS hailed the approval, without which the upgrade between Broadwater and Coolgardie cannot proceed.