While the sun went down, crowds gathered to see teams from Transport for NSW and John Holland begin the huge operation of dismantling the old bridge, a project expected to be completed by December.

The first part of the operation was to push a barge with a low loader under a southern section of the bridge, a task chosen to be completed at low tide.

As the tide rose overnight it helped dismantle the bridge before a crane removed the section from the barge on Thursday.

The well known Merinda river cruise boat experienced it's first trip 'through' the old bridge, captured by Bay local Jenny Kohlhagen.

Crews worked together to ensure the placement was correct and the barge slowly settled into place.

Transport for NSW regional director south Sam Knight said the removal of the truss span would eliminate the need to operate the lift span.

"The removal of this span will allow a 15-metre navigation channel for marine vessels with 10-metre clearance at high tide to be opened temporarily in the following days," Ms Knight said.

"Transport for NSW respectfully asks for patience and understanding from the community while the old structure is removed, as some of the work will take place outside standard construction hours."

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River-based span removal work is set to take place within the marine exclusion zone, bordered by safety buoys and signage.

As work progresses, the marked navigation channel and marine exclusion zones will change, and Transport for NSW asked all water users to keep a proper lookout, comply with local buoyage and signage, and exercise extreme caution when navigating near the works.