Floods, bad weather and tides have put work on the Waihao Box, on Waimate's coast, about six weeks behind schedule.
This has delayed reopening the mouth of the Waihao River and its reserve to the public while contractors continue working on the 104-year-old wooden structure for Environment Canterbury (ECan).
ECan closed the reserve on February 17, estimating it would take about two months for the work to be completed.
The work under way is one of the the most extensive reconstructions to be carried out on the Waihao Box and will improve its effectiveness which had decreased in recent years because of damage caused by regular pounding by heavy seas.
Last month, it was estimated it would cost about $400,000. Now ECan is estimating it could take six weeks or longer to finish, because tides allowed work only every alternate week.
ECan southern area engineer Bruce Scarlett said a combination of flooding in the Waihao River and Wainono Lagoon catchments, heavy swells and bad weather led to the delays.
''The fourth compounding factor was that sometimes we'd have good weather and sea conditions but the tides would be wrong,'' he said.
The Waihao Box is a unique system that allows permanent flows of water from the Waihao River and Wainono Lagoon to the ocean, but a hazardous place for both the contractors on site and the public, which was why the reserve had been closed while work was done.
''There can be big ocean swells there and the beach gravels are continually shifting, so it's a dangerous place already and one that has already claimed one life some years ago.
"Add heavy machinery, and it became essential for everyone's safety to close the neighbouring reserve,'' Mr Scarlett said.
The work involved the replacement of rotting timbers and piles and repositioning the box in response to changes in the beach profile.
The bulk of the cost is being funded by about 30 ratepayers in the Wainono Drainage Group and the remainder by ECan and Meridian Energy.
