The river mouth closes naturally each year, a process the Glenelg Catchment Management Authority says is "extremely important" to fish, birds and native vegetation.

When the mouth of the estuary opens under the wrong conditions, fish deaths can occur from sudden changes in water quality, the authority says.

But Narrawong farmer Owen Roberts said the river reached 1.9 metres above sea level this year making about 10 acres of his farm "unproductive".

"It backs onto farmland and has serious impact on the local camping and recreational reserve," Mr Roberts said.

"It's school holidays, it couldn't be a worse time."

This year Narrawong Holiday Park visitors are driving on a submerged 20-metre stretch of a sealed council road to access their accommodation.

Mr Roberts and community members want a policy overturned keeping authorities from manually removing a sandbar that plugs the river mouth.

But a Glenelg CMA spokeswoman said conditions had "not allowed for a safe and appropriate estuary opening this year".

"We have been subsequently monitoring the situation very closely," she said.

"In particular, low freshwater inflows and very low dissolved oxygen levels in the estuary mean that an opening at this time presents a very high risk of fish deaths."

If rainfall trends continue she said the river should "open naturally and reduce estuary water levels in coming weeks".

Mr Roberts expects the sandbar will break in coming days and the water level could lower in "a matter of hours".

But he believes the force of the river naturally lowering will also kill fish.

"The big thing will be when it breaks we will be looking for dead fish and so will the seagulls," Mr Roberts said.

"That's when a more controlled opening is needed. There has to be a release."

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