The National Capital Authority, which manages federal land in Canberra, told a parliamentary briefing they couldn't offer much to those seeking land for diplomatic missions.

"We've had a number of countries where we've basically said, 'No, we can't offer you anything'," chief planner Andrew Smith said.

He also told the committee negotiations with the Russian and Iranian missions to see them start works on their vacant land was ongoing.

Mr Smith said new embassies could be built for a number of reasons, including the beginning of a new diplomatic relationship with Australia or countries already stationed in Canberra looking to move out of rented premises.

But some countries were also requesting larger premises for missions.

"We are actually at a situation where we really can't effectively offer anything much," Mr Smith said.

He said there were a couple of vacant sites but the authority couldn't offer them due to "reasons associated with terrain or soil conditions".

When contacted by AAP, the authority said it was up to the individual missions to confirm whether they had been turned back.

The authority manages vacant sites but countries then have to look after it once they pick up the lease.

It also has a 'use it or lose it' policy where countries may have leased land taken away from them if they show no signs of developing it.

Mr Smith told the committee negotiations with the Russian and Iranian missions to see them start works on their own missions were continuing.

Last year, the committee was told that Kuwait, Iran, Brunei, Russia and Bangladesh had stalled or inactive development on their land.

At the time, Iran attributed delays to the unexpected death of their embassy's new architect.

It also said it was holding discussions with the ACT government on the shortage of diplomatic land.

Australian Associated Press