The speed limit around the Waterview Tunnel increased to 100km/h on Monday but since then the speed limit has changed between 80km/h and 100km/h several times.

The NZ Transport Agency announced it would be increasing the variable speed limits on sections of State Highways 16 and 20 around the tunnel to allow maximum speeds of 100km/h in certain driving conditions.

NZTA acting system design manager Kevin Reid said speed limits were likely to be reduced during peak periods when there were typically high traffic volumes and in the event of incidents, crashes, maintenance work or extreme weather conditions.

That is why the speed limit is not always 100km/h.

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Variable speed limits meant that the maximum legal speed limit could change between 30km/h and 100km/h and be adjusted for the road and driving conditions.

The first phase would happen on SH16 between St Lukes and Rosebank and on SH20 between Maioro St and Dominion Rd, he said.

The second phase, on SH16 between St Lukes and closer to the central motorway junction, would follow at a later point once further work on signage and system requirements had been finalised.

"The Transport Agency takes adjusting speed limits very seriously and we have appreciated people's patience while the necessary investigations, consultation and legal processes were completed," Reid said.

Reid said it was very important for drivers to watch the motorway signs and be aware the speed limit in the variable speed limit zones will be different at different times of day.

In the weekday morning peak period, the maximum speed for traffic heading into the city on SH16 is 80km/h between Rosebank Rd and St Lukes.

At all other times it is 100km/h.

During the evening peak period, the heavier traffic flow on SH16 is out of the city and the variable speed limit between St Lukes and Rosebank Road is 80km/h. At all other times it is 100km/h.

On weekends and other off peak times the maximum speed limit will be 100km/h.

Speed limits can also be lowered at any time in the event of incidents, crashes, maintenance work or extreme weather conditions.

"There have been no incidents or noticeable changes in driver behaviour associated with the changes," Reid said.

"The motorways around the Waterview Tunnels are complex with a lot of different lane changes and high traffic volumes so we encourage people to follow the signs and drive to the conditions at all times."