Two skyscrapers backed by state Planning Minister Matthew Guy have been scaled down by flight path restrictions over Melbourne.
A CBD office tower proposed by the property tycoon Harry Stamoulis is unlikely to go ahead, and air safety height restrictions forced the developers of another Southbank giant, the Australia 108 tower, to redesign it.
After being approved by Mr Guy in March, Australia 108 was touted - at 388 metres - as the southern hemisphere's highest residential structure, surpassing Melbourne's tallest, the nearby Eureka Tower.
Mr Stamoulis' 404-metre proposal at 555 Collins Street would also have beaten the Gold Coast's Q1, Australia's tallest building, which soars to 323 metres.
Both developers' big plans have crashed to earth following a Fairfax Media report that Australia 108 would infringe federal PANS-OPS regulations that protect air safety.
Australia 108 had encountered regulatory difficulties, developer Benni Aroni said.
''One such condition was the Melbourne PANS-OPS flight path restriction enforced by CASA limiting the tower's potential height to 312 metres,'' he told potential buyers in an email this week. Another reportedly involved VicRoads.
Mr Guy approved a special building envelope for both developments. No plans have been submitted for the Collins Street proposal, which has attracted criticism about overshadowing the Yarra River.
This week Mr Guy said: ''I don't think there was ever a likelihood of a building on a footprint that small being built to 400 metres. The issue at 555 Collins is, and always is, going to be overshadowing the Yarra.''
Air Services Australia confirmed that there was ''no flexibility'' to approve a building that breached height restrictions.
The problem has prompted the state government to seek to change emergency flight regulations for Essendon Airport, the flight path limiting the towers.
The airport's southerly approach passes over central Melbourne and has a 373-metre height limit that extends 15 kilometres from the runway.
Mr Guy said buildings were able to encroach on airspace protected by PANS-OPS and that similar flight path boundaries had been moved in other cities.
Melbourne City Council's deputy planning chairman, Stephen Mayne, said federal authorities needed to clarify the impact of PANS-OPS guidelines.
''It would be good to have some greater transparency about what the flight paths are, and what the practical implications are on Mr Guy's frenetic approval of skyscrapers,'' he said.
The architect Nonda Katsalidis, who designed Australia 108, would not comment. Mr Stamoulis did not return calls.