A planning rule wouldn't bend so the lights at this Canberra oval had to

The Manuka Oval light poles are curved in order to comply with a planning rule for Canberra's Parliamentary Zone. (Supplied: Ben Wrigley)

If the six light poles skirting Canberra's Manuka Oval look like they're on a lean, that's because they are.

The 47-metre-high lights lean inwards, because, if they were standing straight, they couldn't be approved under the city's RL617 rule.

The strict planning regulation wouldn't bend, so the light poles had to.

RL617 isn't a rule that most Canberrans would encounter on a daily basis, but developers, planners and architects know it well. It sets the maximum height for structures in Canberra's Parliamentary Zone.

"When you look across Civic and think it's all a bit stunted … it's because of that," Rodney Moss, former director of Cox Architecture, said.

The height limit under the rule is 617 metres above sea level, which is specifically set to a chosen point on the landscape — the grassy peak of Parliament House's sloping lawns.

The height limit under the RL617 rule is 617 metres above sea level — the same height as the hilltop of the Parliament House lawns. (Supplied: John Macdonald)

Parliament House was designed so that ordinary Australians could walk over its roof, putting the people literally above their parliament.

And this, among other natural vistas in the Parliamentary Zone, is exactly what RL617 was designed to protect.

In 1990, the National Capital Authority (NCA) formalised the height limit as a key urban design rule in the inaugural National Capital Plan, preserving architect Walter Burley Griffin's vision of a bush capital, where the natural environment remains dominant over built form.

In doing so, Parliament House's rooftop hill became the grass ceiling of the city.

Only Parliament House's flagpole and the dome of the Australian War Memorial poke above the invisible line.

The Manuka Oval light heads are shaped like a shovel. (Supplied: Ben Wrigley)

In 2012, when broadcast-quality lighting was being installed at Manuka Oval to enable the venue to host more night-time sporting matches, Mr Moss had a challenge on his hands.

"In order to give you television-quality light on the oval, the lights need to be a certain height," he said.

But Mr Moss said that would have taken the lights above the limit imposed by the planning rule.

"What we did was bend the lights so that they came down below 617 metres above sea level, and then we made the light head into a shovel, so that the maximum amount of light is right on the edge," he said.

Architect Rodney Moss says while the lights' angled design sol