Experts say the environmental water flow agreement for the Snowy River has never reached its annual target.

For decades, the Snowy River was diverted for the Snowy Hydro Electricity Scheme, until in 2002, when the historic Snowy Water Inquiry Outcomes Implementation Deed was signed.

The deed includes a commitment for governments to restore environmental flows.

But now, an Australian National University review published by the CSIRO says in the 20 years since the signing, the annual target for flow restoration has not been reached. 

Only about two thirds of the targeted environmental flows have been returned over that time.

One of the major problems highlighted by the scientific review is a lack of a single government agency responsible for meeting the targets.

Additionally, the deed itself has no clause in it for review.

A local advocacy group, the Snowy River Alliance, says the external review shows that the agreement lacks transparency and is inadequate.

The group is calling for a formal NSW Government review into the deed agreement.

The full study is accessible here.