US prosecutors have won a court case that brings them a step closer in their attempts to have the Megaupload founder and three of his colleagues extradited from New Zealand.

The appeals court in Wellington overturned an earlier ruling that would have allowed Kim Dotcom and the others broad access to evidence in the case against them at the time of their extradition hearing, which is scheduled for August. The four are accused of facilitating massive copyright fraud through the internet filesharing site.

The court ruled that extensive disclosure would bog down the process and that a summary of the US case would suffice.

Dotcom, a German national, says he is innocent and cannot be held responsible for others using the site to illegally download songs and films. Along with him, US prosecutors are seeking the extraditions of Finn Batato, Mathias Ortmann and Bram van der Kolk, each of whom held senior positions at Megaupload before American authorities shut the site down in 2012.

Paul Davison, one of Dotcom's lawyers, said he planned to appeal at New Zealand's supreme court. Dotcom's legal team must first submit an application to the court which will then decide whether an appeal has enough merit to proceed.

In its ruling the appeals court found that full disclosure of evidence was not necessary at the extradition hearing because the hearing was not the venue to determine guilt or innocence. The court pointed out that the legal obligation on the US was simply to prove it had a valid case.

The court also found that extradition treaties are essentially agreements between governments and "even though courts play a vital part in the process, extradition is very much a government to government process".

Davison said it was vital that Dotcom had access to a wide range of documents including those that could be detrimental to the US case. The lawyer said that would help prove there was no merit to the case.

The extradition hearing has already been postponed from March to August owing to the legal wrangling. It could be postponed further should the supreme court decide to hear the next planned appeal.

Dotcom remains on bail pending the hearing. In January, on the anniversary of his arrest, he launched a new filesharing site called Mega.