A massive electrical power failure has crippled the Australian Navy's largest warship as it completes its humanitarian mission to Tonga following the January 14 volcanic eruption and tsunami.

The ABC can reveal that despite days of emergency work HMAS Adelaide, known as a landing helicopter dock (LHD), is still experiencing problems, having earlier been "stranded".

An email sent by HMAS Adelaide's commander states: "Marine Technical Department have worked throughout the past few days to restore power and get us operational again." 

In the message, obtained by the ABC, Captain Stuart Watters says: "We are still experiencing issues with external communications and wanted to let you know that everyone onboard is safe.

"We want to thank the technical team for their hard work in hot and difficult conditions which is enabling us to complete our task and restore services and communications."

Sources on board HMAS Adelaide have told the ABC the ship was "stranded" for a period of time, because of a "total power failure", including the backup power.

According to several members of the crew, most of the ship's company were now sleeping "above deck" because of the power problems.

Last week, HMAS Adelaide was allowed to dock in Tonga to deliver humanitarian supplies, despite a COVID-19 outbreak that occurred on board shortly after it had departed Australia.

The ship's commander insists "COVID protocols are in place and effected personnel are being provided with medical care and being supported".

"A number of people who were originally identified as infectious have been cleared to return to duty after completing quarantine," Captain Watters wrote.

The Defence Department is yet to respond to the ABC's request for comment.