Mr Penrose was heartened when Mayor Brown told him and Manurewa councillor Sir John Walker that KiwiRail believed a southern link could be provided for $5-6 million.
However, Mr Penrose feared that estimate might be "a little bit on the light side".
Council safety forum chairman George Wood said it seemed "totally inefficient" to have built the new line without a south-facing connection, requiring people heading that way from Manukau to change trains after travelling little more than 2km.
KiwiRail would not discuss prospects for a southern link or confirm a suggestion that Ports of Auckland's new railhead at Wiri may be in the way.
Infrastructure and engineering general manager Rick van Barneveld said it had not received a proposal from Auckland Transport to build such a link.
Despite the mayor's promise, Auckland Transport spokeswoman Sharon Hunter said no provision for a southern link was being included in a network plan under development by her council-owned organisation.
Mr Wood is also concerned that the new Manukau railway station ends in a trench "in no-man's land", 400m short of the bustling Westfield shopping mall.
Auckland Transport hopes the Manukau Institute of Technology's development of a $94 million multi-storey building above the new station, with a $13.6 million council contribution, will help to boost annual patronage to 600,000 passenger trips.
But rail services are starting modestly, with three trains an hour at peak periods in each direction between Manukau and Britomart, and hourly at other times.
The trips will take about 40 minutes, mainly via Glen Innes on the eastern line, although occasionally through Newmarket.
MANUKAU LINE
* 2km from a 300m trench west of the Manukau civic centre to the main trunk link at Wiri.
* Cost: $81 million for first stage followed by $94 million Manukau Institute of Technology building.
* Train services: three an hour in each direction at peak periods; hourly at other times.
* Trip time about 40 minutes.