Inaction by a West Australian port authority may have accelerated the deterioration of one of the state's oldest homes, according to an internal report obtained by the ABC.

The heritage-listed Leschenault Homestead, about 160 kilometres south of Perth, was one of the earliest residences constructed in the South West after colonisation.

It was sold to the Bunbury port authority, now known as Southern Ports, in 1968.

Significant and progressive degradation has been consistently observed and flagged by local residents and councillors in recent years.

Architect Kent Lyon's report highlighted the ongoing deterioration at the homestead. (Supplied: Kent Lyon)

In May 2024, Southern Ports told a community committee the cottages had "been reviewed by Kent Lyon, who will provide a report on the stabilisation of the buildings". 

In August 2024, the same committee was told "a local heritage architect is working on steps on how to stabilise the buildings".

When the ABC asked Southern Ports for a copy of this report last year, a spokesperson said it would take some time to source.

After a Freedom of Information application by the ABC, a copy was released. 

'Urgent' work required

Completed in April 2023, the report recommends a range of measures, several requiring urgent attention and the remainder requiring attention within 12 months.

Urgent work included removal of plastic sheeting placed over the eastern wall of one cottage, which Mr Lyon said had "exacerbated the deterioration [of the weatherboards] with a humid 'glasshouse' effect". 

A photo from Kent Lyon's 2023 report shows damage to one of the cottage's bases.  (Supplied: Kent Lyon)

Southern Ports said it had installed the black plastic "with good intent; to preserve and weather protect the interior of the building".

Almost three years later, this plastic sheeting remains in place.

The port authority said the conservation works should occur as a package, and the removal of the plastic sheeting was contingent on cladding being installed.