A Murray-Darling Basin Authority manager on Friday announced the road crossing would close in July 2021, after having been slated to close in 2020 since 2003.

Head of river management Andrew Reynolds said the delay was due to roads bodies having to upgrade the Yarrawonga-Muwala bridge for extra traffic when it became the towns' sole crossing.

The work will involve the bridge being closed for periods.

"We talked through with them whether we reopened the weir bridge periodically but we agreed the disruption would be quite significant and it would be better to leave it operating until works are complete," Mr Reynolds said.

Transport for NSW regional director Lindsay Tanner said "we are yet to finalise the work schedule but anticipate starting to repair the bridge's steel truss and upgrade traffic and pedestrian barriers in April or May 2020".

"This work, which is jointly funded by Regional Roads Victoria, is likely to be completed by the end of June 2021," Mr Tanner added.

Closing the single-lane weir bridge will see grain carriers and trucks travelling to Mulwala's munitions factory use the main crossing.

The 1939 weir is the only one on the Murray River that motorists can drive over.

"The staff from Goulburn-Murray Water that actually operate Yarrawonga weir, four staff, they're out every day undertaking maintenance activities and of course having traffic incoming raises the safety risks to workers," he said.

"This is about making sure we keep pace with contemporary safety standards."

Mulwala-based Federation Shire councillor Bronwyn Thomas welcomed the MDBA setting a closure date, noting it came after she called for detail from the body at a meeting this week.

She hopes it will provide some impetus for progress on building a new Yarrawonga-Mulwala bridge.

It was anticipated in 2003 there would be a fresh span over Lake Mulwala by 2020.