Board chairman Darren Ludlow said the objective was to ''create something that sees our museum hold its own as a leading place for heritage, culture and education''.
The distinctive pyramid building was no longer large enough to house the collections and all the services required, such as the cafe, education centre and tourist facilities, he said.
The original redevelopment proposal in 2007 was estimated at $24.6 million, but was later downsized to $20 million in a bid to reduce costs.
The project was further delayed last year after a seismic assessment found the facility's four buildings were an earthquake risk.
The board proposes to undertake the new redevelopment in two phases over 12 years, starting in 2017.
The first phase, costing $20 million, would be a new building west of the pyramid, while the second phase involved refurbishing the pyramid.
Grants would be sought from the Government and community funders, Mr Ludlow said.
The Invercargill City Council has already committed about $3.5 million towards the project and the Community Trust of Southland $5 million. Those funds would go towards phase one, he said.
The redevelopment plans include space for a $1.98 million facility for kakapo chicks and tuatara which the city council proposes building next to the pyramid, but its cost is not part of the trust board's budget.
Earlier this month city councillors approved the kakapo facility in principle.
