"Be brave, just work with us. Don't be frightened, don't be afraid of us, work with us."

In particular, Greg wants Main Roads WA and the state government to work with traditional owners on a lower-impact design for the proposed Lloyd Street Bridge over the Helena River wetland between Midland and Hazelmere.

Community groups and traditional owners say in its current form the bridge will damage the wetland and a registered Aboriginal heritage site.

As an elder, Greg said his responsibilities include speaking for his community, standing up for wildlife, the landscape and heritage. On behalf of 12 traditional owners, he has written to federal Environment Minister Sussan Ley, who has agreed to assess the development under the Aboriginal Torres Strait Islander Heritage Protection Act 1984.

"The floodplain is the mythological site for the Wagyl, the Noongar creation spirit, and is culturally very important to us," Greg said in his submission. "Any disturbance of this site will cause irreversible damage to its heritage and mythological value."

The project, co-funded by the state and federal governments and the City of Swan, has approval under Section 18 of the Aboriginal Heritage Act 1972 - (the Juukan Gorge rock shelters, destroyed by Rio Tinto, came under the same act).

Greg said during the consultation process, traditional owners were told they needed to be pragmatic and accept a bridge would be built. It will create a new entry to Midland, provide better access to St John of God Hospital, the Curtin University campus and an emerging entertainment precinct.

"We will accept a bridge but only if it is a low-impact design that protects important site features including a rock shelter and a seasonal nesting pool," Greg said. "If a bridge must be built, then it should be the widest bridge possible that has the least impact on the river and floodplain."

Greg's letter said the current design means the floodplain would be buried under 100,000 cubic metres of infill. More than 60 mature trees, including scar trees, would be removed; a seasonal pool - habitat of the long-necked turtle - would be buried; and an ancient rock shelter impacted.

A report to the City of Swan Council said Main Roads WA had advised a redesign would cost "tens of millions of dollars".

A spokesperson for Ms Ley said an independent body would prepare a report to support the assessment of the section 10 application. "The minister will then assess the report before making a determination." Indigenous Australians Minister, local MP Ken Wyatt, said it would be inappropriate to comment while the assessment was being undertaken. A spokesperson for Transport Minister Rita Saffioti said subject to design finalisation, early construction activity is scheduled to begin in the second half of the year.