Once a harsh critic of the $344 million Sydney Modern expansion of the Art Gallery of NSW, Paul Keating played a crucial role shaping the final plans backed by the state government.

The former prime minister's involvement has also has left ajar the possibility of realising a vision he has long held: demolition of the Cahill Expressway to reunite the Domain with the Royal Botanic Gardens.

Former prime minister Paul Keating played a crucial role shaping the final Sydney Modern expansion plans backed by the state government. Credit:Eddie Jim

On Wednesday, Fairfax Media revealed the state government had given the go ahead for the Sydney Modern expansion via $244 million funding in next week's budget.

A further $100 million, of which $70 million has been pledged, is being raised by private donors. Construction is due to start in 2019 and be completed in time for the gallery's 150th anniversary in 2021.