Mayor Len Brown wants the project started in 2016, despite having yet to find a way to fully fund the city's share.
But Mr Townsend said: "My personal view is that wanting to speed it up and whip it along is possibly a bit naive because a vast amount of consultation and planning has to be done."
He hoped Auckland Council as a whole would work with businesses near the project to minimise disruption to custom during the five-and-a-half years it would take to complete the project, which will include a "cut-and-cover" trench along Albert St to an underground station near Aotea Square.
Although deep tunnels will be bored with little above-ground impact, about 75 surface properties will be needed.
Auckland Transport has accepted a requirement to prepare management plans to limit social and business disruption and will have to limit intersection closures to east-west traffic along Albert St to one at a time.
But a spokeswoman said an "informal" recommendation that it provide an easy compensation scheme for businesses for lost custom during construction was better left to Public Works Act procedures.
Affected property owners, once notified formally of her organisation's decision, will have 15 working days to decide whether to appeal to the Environment Court.
Heart of the City business organisation chief Alex Swney said he suspected the potential construction impact on some businesses had yet to sink in.
But he welcomed Auckland Transport's promise of prompt mitigation.
"Too many infrastructure projects have been built where a public benefit has been delivered at crippling private cost."
A spokeswoman for MediaWorks, whose headquarters including TV3's studios are close to the southern end of proposed twin rail tunnels, said it fully supported the project with its offer of better public transport but remained concerned about the impact on its business from noise.
It was "taking further advice on whether the conditions are workable for our business".
