The park at Blanchetown, northeast of Adelaide, was swamped on Monday after the Murray broke its banks nearby over the weekend.
Park manager Cassie Powell said the water rose very fast.
"So to anybody out there on the river, if you think you have another day, you more than likely don't," she said in an emotional social media post on Sunday.
"We thought we had another day going by the front of the river but when it broke the bank the water was spewing into the backwaters like a big Niagara Falls."
Ms Powell said the water had completely inundated the park, including its shop and office, with the site now closed.
Her warning came as the State Emergency Service began installing the final series of flood barriers at Loxton and Berri.
So far about 4km of the DefenCell technology has been deployed along the river, the system using large interlinked bags filled with soil.
"The DefenCell product can be deployed much faster than traditional sandbags, in particular when it is used to protect large areas,'' SES chief of staff Derren Halleday said.
"Over the weekend a large wall was completed in the main street of Mannum, allowing us to move onto the townships of Berri and Loxton."
Up to 4000 properties in SA are forecast to be inundated by the water surging down the Murray, with more than 450 of those considered permanent residences.
The first peak in the river level is tipped to reach Renmark, near the Victorian border, about Wednesday.
The peak is expected to reach Mannum, east of Adelaide about December 27.
After a brief settling of water levels, a second and higher peak is forecast to reach the SA border towards the end of the year.
That could result in daily flows of up to 220 gigalitres.
So far more than 5000 properties across the region have been doorknocked to check on flood preparations, including 1100 that were still occupied.
The rising waters have prompted SA Power Networks to cut electricity supplies to 1850 properties with more disconnections likely.
The Lyrup, Mannum Upstream, Morgan and Swan Reach ferries are closed, with the ferry at Walker Flat also expected to close.
The Waikerie ferry has moved to a higher landing.
The SA government has provided a $51.6 million assistance package, including support for tourism and other businesses, as well as direct emergency payments to homeowners.
Last week it allocated an extra $1.2 million for emergency accommodation.
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