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St Clair resident Barbara Newton said she and her husband were standing on the beach when it happened.
''We saw her and thought 'what the hell is that woman doing on the beach', and then my husband noticed the little dog [get] washed away.''
The woman came out of the water safely, but ''distraught''.
The dog, which is still missing, has been described as a 15-year-old Jack Russell terrier.
About a dozen people searched for the dog yesterday after the incident, to no avail.