But the National Portrait Gallery will close for six months in 2019 to deal with structural problems in the building, barely a decade into its Parkes tenure.

Director Angus Trumble has confirmed that due to unavoidable maintenance works, the gallery will close from late April to September next year.

"We have to replace a brace of windows, in which the double-glazing is faulty, we have to replace the floors throughout the galleries, and we have to replace the waterproof membrane which underlies the concrete podium that surrounds the building," he said.

"We looked at trying to do it in stages, that would enable us to stay open, but when you think about it in terms of circulation, and visitor experience, what we decided to do was bite the bullet and do it all at once, in as compressed a period as we could."

The $87-million building, designed by Sydney architects Johnson Pilton Walker, was opened by then prime minister Kevin Rudd in 2008, and has since won more than 20 awards from the design and construction industry.

Mr Trumble said gallery staff first discovered water leaks in the underground car park after torrential rains around two years ago.

"The only conclusion we could draw from that was that the membrane underlying the concrete was perforated, so there's no solution, other than to replace it. So that's that," he said.

"We've ascertained that the collection is safe, and will be safe, but obviously if water is getting into the underground carpark, there's a long-term consequence if you let that continue in terms of further deterioration over time. So it really does need to be dealt with. It's one of those things where you just can't not do it."

He said it wasn't possible to say how much the repair works would cost, as the project had yet to be put out to tender, a process that would likely take place towards the end of the year.

Chairwoman of the gallery's board Helen Nugent said the the decision to close had not been made lightly.

"If this decision hadn't been reached, it would have gone on for a lot longer, so this was the shortest way of ensuring that there was a quality visitor experience," she said.

While the gallery spaces, shop and cafe will close, gallery staff would be able to remain in the administration wing throughout the six months.

During the closure, the gallery plans to ramp up its national touring exhibition program, and will have portraits on show at various locations around Canberra.

"We are going to have a footprint elsewhere in Canberra. Those discussions are underway, and are yet to be confirmed and we'll make an announcement as soon as it is," he said.

"As you can imagine, it's very rare to have an opportunity, a hole in the program in which you can undertake completely different programs when you have the time. So we're going to ramp up our online learning capacity, in a big way, and we are going to send the troops with the touring exhibitions into the venues where we're sending shows, and deliver programs in the field. It's rather exciting, it's like natural history."

Mr Trumble said the reopening of the gallery in 2019 would also mark the launch of the recently announced Darling Portrait Prize, a new contender in the already highly populated crowd of portrait prizes in Australia.

"It is different [to other major prizes], but we've looked very carefully at other models and we think that we've come up with a really good one that borrows from the best of the best all over the world," he said.

"It is a way of generating new growth, so in terms of there being a number of other existing prizes, it seems to me it's good for artists."