"Increased work activity will mean that there will be some disruption to services."
Warmer weather also meant an increase in heat restrictions, Mr Shepherd said.
"When steel rail is heated it expands, and, in some cases, can cause the track to distort out of alignment, so speed restrictions are put in place to ensure safe running.
"Temperature sensors will detect when the rail goes above 40 degrees celsius and there will be 40km speed restrictions for trains in these areas.
"Just like a car on a warm day, the metal captures and retains heat, and can reach temperatures far in excess of the surrounding air temperature.
"Therefore rail operators around the world will put heat restrictions in place to prevent damage, or even buckling to the track."
There are 14 locations between Wellington and Masterton where heat restrictions could be put in place.
Speed restrictions were needed to keep staff and passengers safe, Mr Shepherd said.
Services on the Wairarapa Line would continue to operate the current timetable but customers should plan for delays.
The work is part of the Wellington Metro Upgrade Project, in which additional funding of $2.3 million has been designated for more track work on the Wairarapa and Hutt Valley lines.
The work includes level crossing upgrades, ballast upgrades and some track component renewal.
This is on top of the $3 million for normal renewal.
