Investigators have discovered more human remains and machine-gun-like holes in a section of wreckage from the downed Malaysia Airlines flight MH17, a week after the airliner was shot down over eastern Ukraine.

Two Australian diplomats and a forensic expert understood to be part of the Australian Federal Police, joined Dutch investigators at the crash site near the hamlet of Grabovo, in the rebel-held Donetsk region.

Michael Bociurkiw, spokesperson for the Organisation for Security and Cooperation in Europe (OSCE), confirmed that key pieces of evidence that had been previously unaccounted for had been located.

"We saw remains yesterday and we saw one spot of remains today," he said, adding that the discovery was an "extraordinary finding".

The newly-discovered wreckage includes a large section of the fuselage of the Boeing 777, with windows and seats still attached.

OSCE says this is the biggest piece of #MH17 found so far. Part of fuselage in a forest. Some windows intact. pic.twitter.com/SwXR2abTmR — Phil Black (@PhilBlackCNN) July 24, 2014

Bociurkiw said sections of the wreckage was punctured with "almost machine-gun types of holes".

It was discovered in an area of woodland more than a kilometre away from the main crash site and is the largest intact piece of the aircraft found so far, as reported by Au News.

"The Australians are getting a sense of the security for the area, they're mapping it, they're getting a sense of where the crash sites are," he said.

Expressing frustration at the slow rate at which foreign experts are visiting the site, Bukiurkiw added: "We're recording a lot of random fuselage and personal belongings. It is striking, even for hardened professionals, to encounter a section of the aircraft that you don't recognise and then there is a passenger's seat and a child's game."

About a mile from the main #MH17 crash site, inspectors came across a huge piece of fuselage they haven't seen before pic.twitter.com/CD7P2BkDDT — CBS Evening News (@CBSEveningNews) July 24, 2014

Another OSCE official told the Sydney Morning Herald that sections of wreckage showed that extreme heat had melted parts of the fuselage: "Some of the finds over there are extraordinary... and the Malaysians who were with us observed that the heat was so intense that the aluminium wing sections of this 17-year-old aircraft actually melted."

Flight MH17 crashed in an area held by pro-Russian separatists on 17 July while on route from Amsterdam to Kuala Lumpur, killing all 298 passengers and crew on board.

Countries directly affected by the disaster, including the Netherlands which lost 193 citizens in the tragedy, have expressed concern that the crash site has not been adequately secured, with farmers driving combine harvesters in areas which could contain evidence or human remains.

Western nations say there is growing evidence that pro-Russian rebels shot down the plane using a surface-to-air missile supplied by Russia, while Moscow has suggested Ukrainian government forces are to blame for the crash.

Australian Prime Minister Tony Abbott has announced a further 100 Australian Federal Police officers will be deployed to the Netherlands, who are leading the investigation, ahead of any mission to help secure the crash site.