An emergency declaration was made at Donnybrook about 6.44am, with boundaries established on Ann Street, Maud Street, Fisherman Drive, Alice Street and Mary Street.
Police were called to an address within the exclusion zone about an hour earlier, with specialist police currently negotiating with a man inside a property.
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The public are advised to avoid the area and those within the exclusion zone are asked to remain indoors until further notice.
The declaration was made under the Public Safety Preservation Act.
