Australia's newest warship is undergoing emergency mechanical repairs to address "engine defects", at the same time the navy's only other supply class vessel remains out of service.

The ABC can reveal HMAS Stalwart, which was commissioned into service less than three years ago, has extended a port visit to Darwin as Defence investigates the extent of problems on board.

HMAS Stalwart is the second and final Auxiliary Oiler Replenishment (AOR) ship built in Spain for the Royal Australian Navy, and is supposed to carry fuel, ammunition and other supplies for deployed combat forces, as well as providing logistics.

Last year the ABC revealed significant mechanical problems had also been discovered on board her sister ship, HMAS Supply, which was taken to Sydney's Garden Island naval base to be repaired under warranty. 

During a Senate estimates hearing earlier this month, senior navy figures revealed that HMAS Supply could remain out of service for another year despite already being sidelined for 14 months.

"I do not want the committee to get the wrong idea, I am not satisfied with the availability of this ship," Navy Chief Vice Admiral Mark Hammond said when questioned about the AOR ship built by Spanish company Navantia.

"This is a complex defect. Navantia have written to the department accepting liability, it will be repaired under warranty," the navy chief confirmed during his evidence on June 6.

Following questions from the ABC this week, the Defence Department has now confirmed that HMAS Stalwart, which was commissioned into service seven months after HMAS Supply in November 2021, is also experiencing problems with its engine.

"HMAS Stalwart has extended a port visit to Darwin to investigate engine defects that emerged on passage to Darwin. The crew are supported by contractors," a Defence spokesperson told the ABC.

"The extent and cause of the required repairs are under investigation," the spokesperson added.

Defence figures familiar with the situation claim HMAS Stalwart is experiencing "massive engineering issues" and could be in as "bad shape" as HMAS Supply.

"It's having to undergo emergency repairs to allow it to limp back to its home port in Perth," one Defence source told the ABC, speaking on the condition of anonymity.

Spain's Navantia is one of several international defence companies shortlisted to supply Australia's new $7-10 billion fleet of general purpose frigates for the navy.

The Spanish shipbuilder has previously supplied the navy's three Hobart-class air warfare destroyers, as well as the two Canberra-class landing helicopter docks, which have also faced long-running problems with their propulsion systems.

Last year the ABC also revealed that sailors serving on board HMAS Supply and HMAS Stalwart were being forced to drink bottled water after a contamination problem linked to plumbing was discovered on board the ships.