Victorians have narrowly backed a new rail tunnel under the city over the Napthine government's proposed east-west road connection.

An Age/Nielsen poll found 47 per cent of voters backed the metro rail tunnel from South Kensington to South Yarra over the $6 billion to $8 billion road project connecting the Eastern and the Tullamarine freeways.

But the results were finely balanced, with 43 per cent favouring the road project, which the government says will begin towards the end of next year.

The rival plans are set to be a key election issue in Victoria, with Prime Minister Julia Gillard backing the rail option and Opposition Leader Tony Abbott promising to help fund the road tunnel.

The finding came as Premier Denis Napthine predicted ''thousands'' of motorists would use the road tunnel daily. But Dr Napthine declined to give details about the expected patronage, arguing it formed a key part of the government's confidential business case, which is expected to be submitted to Infrastructure Australia in coming days.