Whale Oil blogger Cameron Slater has to pay about $15,000 costs to news companies he tried to stop from publishing emails linked to the Dirty Politics saga.

Slater took legal action against APN, Fairfax NZ and MediaWorks to stop them publishing material obtained from his electronic storage media.

The action followed the publication of Nicky Hager's book Dirty Politics, information for which was sourced from emails hacked from Slater's computer.

An application by Slater for an urgent "interim interim" injunction heard in the High Court on September 4 was unsuccessful.

Slater filed an amended application and on September 9 the court was told an agreement had been reached under which the news companies promised not to publish personal information about family or medical matters.

Slater discontinued proceedings on September 29 and Justice Mary Peters recognised that that discharged the companies from their undertakings.

A reserved judgment on costs by Justice Simon Moore was delivered today in the High Court at Auckland.

He said APN and MediaWorks were each entitled to $5039.51 costs, and Fairfax was entitled to $4542.01, with disbursements of $500.

The amount for each of the defendants was consistent with the schedule of time allocations and appropriate daily recovery rates, he said.

Lawyers for the news companies had sought a 30 per cent increase in costs because of the complexity and level of skill required, but Justice Moore said he was not satisfied they were warranted.

Legal principles governing the publication of hacked material were not well established, he said.

The judge who heard Slater's application, Justice John Fogarty, had refused an interim injunction against the defendants, clearly concluding the public interest prevailed, Justice Moore said.

"However, he also noted the difficulties of what remedies the plaintiff had once the information was released into the public arena. Given these tensions I am not of the view that increased costs are appropriate."

Arguing for the increased costs, the defendants had submitted they had clear defences available, and the principles involved were of very significant public interest in the context of the general election campaign.

Slater did not accept the companies were entitled to costs, given the undertakings that had been given.

He would not comment on the ruling today.