The Waihao Box reserve, closed for about three times longer than anyone expected, will reopen again this weekend after the completion of major maintenance on the wooden flood control structure.
Environment Canterbury (ECan) closed the reserve and area surrounding the box at the river mouth when work started on February 17, because the construction work would have made the area unsafe.
The work - costing an estimated $400,000 and the most extensive in the box's 104-year history - was needed to improve the structure's effectiveness after it was damaged in recent years by regular pounding by heavy seas.
Originally, it was expected the work would take about two months, but several factors caused delays and kept the reserve closed for longer.
However, it will reopen again on Saturday.
ECan southern area engineer Bruce Scarlett said yesterday excellent progress had been made on the project in recent weeks after bad weather, flooding and heavy seas had slowed work down early this winter.
''We have had a reasonable run and the Waihao Box is now back working as intended to provide effective drainage for the Waihao River and Wainono Lagoon catchments.''
The work involved replacing rotting timbers and piles and repositioning the box in response to changes in the beach profile.
''We're confident that the box will now be operating well for many more decades to come,'' Mr Scarlett said.
