Honiara's water supply may be switched off, with the Solomon Islands Government saying it can't afford to pay a bill from the land trustees.

The land trustees of Kongulai on Guadalcanal manage the regions from which Honiara's water supply is drawn, and distribute the payments among the tribes in the region.

They say the government still owes them $SI6 million ($US850,000), and have negotiated an agreement under which the government has until lunchtime on Wednesday to pay $SI4.9 million ($US700,000), or they will switch off the water supply.

The Permanant Secretary of the Ministry of Lands, Stanley Waleanisia, revealed that this had been an old debt from five years ago.

"I explained the difficulties of making a payment, given there is no budget for it," he said.

"I made it clear to the trustees that we made a supplementary to the parliament and we're waiting for the parliament to meet, maybe later this year."

He says until the parliament meets - the government can only afford to pay $SI300,000 ($US43,000) - less than a tenth of what is owed.

It's not a hollow threat as the landowners turned the water to Honiara off for two weeks last December after the government fell behind in its schedule of payments.

The government has paid one installment of $2 million on that occasion, but landowners now say they are waiting for two further payments.

Damaso Roko, one of the elders of the Tanadai Haubata Tribe overseeing the payments says if the government pays up, the crisis can be averted.

"If they complete the payments, then I will make a new agreement and transfer the lease agreement to the water authority, so that I will have no problems on the payment," he said.