It intends to install a new signalised crossing for cyclists and pedestrians making their way between two completed sections of the 1.9km bikeway, which will ultimately run from Upper Queen St to Beach Rd.
The agency is also adding an extra right-turning lane at the bottom of Alten Rd, giving vehicles easier access to the Northwestern Motorway via Stanley St.
Agency highways manager Tommy Parker said the temporary road closure would allow his team "to get in and do the work as quickly and safely as possible so that everyone can benefit early".
Although the Transport Agency's original target for completing the cycling "super highway" was December, Mr Parker said contractors were racing to open the off-road facility in September, when it will pass below Grafton Bridge and include a tunnel under Wellesley St East.
The route is an extension of the Northwestern Cycleway from Henderson.
* Visit tinyurl.com/nzcyclesafe to read our cycle safety series.
Editor's note: article updated 23/01/14