Wilton said motorists travelling straight through the roundabout towards Tauriko no longer needed to share a lane with right-turning vehicles.
"These changes will improve the flow of traffic travelling straight through the roundabout.
"Please take extra care when navigating the roundabout during the first few weeks, as people adjust to the changes."
Contractors will be completing minor finishing works over two nights next week.
The Hairini Safety Improvements project is part of a wider programme of work to improve the SH29A Maungatapu and Hairini roundabouts.
Minor changes were made to the Maungatapu roundabout in January this year to improve safety and traffic flow, and to make signage clearer.
Last month the Maungatapu roundabout and the corridor leading up to the Hairini roundabout were resealed under Covid-19 alert level 4 and level 3 restrictions.
The Hairini roundabout is currently scheduled for resealing in September 2020.