More than 150 Sri Lankan asylum seekers detained on an Australian Customs vessel are being given only three hours of natural light a day to eat meals, the government has told the High Court.

It comes as the judge presiding in the case has called for an end to their detention at sea.

Lawyers for Immigration Minister Scott Morrison tabled previously-secret defence documents on Tuesday.

The documents reveal the number of detained asylum seekers is more than previously stated, with 157 instead of 153 on board.

They also reveal the passengers are being given approximately three hours outside in natural light for meals, subject to weather.

That's because it would be "unsafe" to give the group, who include children, unrestricted freedom to move about the vessel, government lawyers argued in the papers.

The documents also show asylum seekers are being kept in one of three rooms onboard the ship, with families separated.

The government says it is maintaining the appropriate separation of men, women and children on board.

There's been no decision the group will be detained at sea indefinitely.

The asylum seekers have been detained on the high seas, outside Australia's migration zone, since their boat was intercepted on its way from India on July 7.

Their lawyers argue they were within Australian waters when intercepted 27 kilometres from Christmas Island, and the decision to deny them entry did not fall within the bounds of "legal reasonableness".

But the government says it has the legal authority to prevent non-citizens from entering Australia.

Meanwhile, a request by government lawyers for more time to prepare their draft cases has been met by frustration by the presiding judge in the case contesting the asylum seekers' possible return to Sri Lanka.

In a directions hearing in Melbourne on Tuesday, Justice Kenneth Hayne granted the extension until Wednesday afternoon, warning both sides that time was "leaching away".

"How long have these people been on this boat," Justice Hayne said.

"This has got to get to an end. Enough."

Commonwealth barrister Stephen Donaghue QC told the court the asylum seekers had been onboard the ship since June 29.

The matter will return to the court for a further directions hearing at 3.15pm on Wednesday.