"Our education programmes have been affected, because noise makes it difficult to take school groups in the native plants area."
As a result, the children get a briefing in rooms on the opposite side of the gardens before heading down to the forest to look for plants and bugs.
Mr Hobbs is concerned about the impact of an extra southbound lane being added to the present three lanes in the next two years.
Parts of the new lane will be elevated above the gardens, bringing traffic closer to the garden boundary.
Mr Hobbs said a screen of 30-year-old shrubby native plants on the native plant area boundary near Hill Rd must be removed to make way for the lane, which meant visitors would see trucks as well as hear them.
"We will need a replanting programme for the visual screen -- it does help if you can't see the cars."
Talks were being held with the NZ Transport Agency about options for some sort of noise abatement in the native plant garden.
The agency proposed to install a 1.1m-high concrete safety barrier along the eastern edge of the new lane adjacent to the boundary, which Mr Hobbs doubted would be enough to dampen noise.
However, the agency disagrees after asking noise consultancy Marshall Day to look into the impact of an extra lane. Its report says the existing six lanes, without noise barriers, produce 64.1 decibels at the native plant garden. With the extra lane and a safety barrier, it predicts the seven lanes will produce 61.4dB.
The report says that with the extra lane, children in the native plant area would experience a 3dB lower noise level, pointing out that a 3dB change is only just discernible.
The report says the noise level reduction would be mainly due to having a solid safety barrier.
