The traditional lands of the Birrabirragal people, the park was designed by John Frederick Hilly, James Barnet and the Office of the New South Wales Government Architect.

From the 1890s, pressure built to buy back private land, and following agitation by the Harbor Foreshores Vigilance Committee (sic), the New South Wales Government took control of 9.3 hectares (22.9 acres) of the Vaucluse estate on 6 July 1910.

The house was then leased by a succession of distinguished persons including Lt. Col. J. G. N. Gibbes, Collector of Customs, Fitzwilliam Wentworth, Attorney General William Bede Dalley, Premier Sir John Robertson and Lady Isabella Martin.

A light AA gun was placed at Steele Point, air raid shelters were built and the 61st Anti-Aircraft Searchlight Company (an all-women unit) occupied the area between the Gardeners Cottage and the avenue of small fig trees to its south.

The large curved masonry stuccoed Roman Seat was erected in 1927 by the Nielsen Park Trust, with a plaque honouring William Albert Notting, who was instrumental in having the reserve established through his involvement with the Harbour Foreshore Vigilance Committee.

Built to a Government Architect design using stone walls but in a contemporary architectural idiom, it was discretely sited at the on the hill at the western end of the beach behind vegetation.

The building was constructed from cement rendered and painted brickwork walls, recessed externally and capped with narrow pitched terracotta tiled roofing on a timber framework.

Nielsen Park has had its pre-settlement landscape dramatically altered by extensive clearing and modifying of landforms and vegetation but also by replanting and regenerating a form of natural bushland on parts of the site.

These are given some added interest by the remnants of the effect of a basalt dyke that runs from Mount Trefle to Bottle and Glass Point resulting in the unusually formed rock formations still visible.

The native vegetation consists of tall heath along the western foreshore containing Allocasuarina portuensis, Smooth-barked Apple and Port Jackson Figs associated with the various exposed sandstone outcrops.

A noted horticulturist of the time, Thomas Shepherd, held that in such a style the lawn should be bold and sweeping, and enclosed on both sides by groups of trees, leaving an open park in front of the house.

The early landscape layout of Greycliffe appeared to have been influenced by this philosophy; the sandstone outcrops and harbourside location were almost made-to-measure natural elements enhancing the "picturesque."

[1] This small sandstone cottage with terracotta tiled roof (as did Greycliffe) was probably also built in 1851 for John Reeve to a design by J. F. Hilly in the picturesque Victorian Gothic Revival style.

There are also other parts of surviving fittings such as timber door frames, glazed brick vents, brass fixings, terracotta pipe drains and traces of white lime wash to walls and some black stencilled lettering.

[1] This is a single storey building or enclosure, dating from 1932, designed in a restrained Inter-war Mediterranean style popular in the 1930s, to provide change and shower facilities for paying visitors using the beach.

The building is constructed from cement rendered and painted brickwork walls, recessed externally and capped with narrow pitched terracotta tiled roofing on a timber framework.

The present structure is in the form of a high retaining wall, behind which are areas of mown lawns abutting the Notting Parade pedestrian and service vehicle road.

Elsewhere the beachfront wall and terraces are in fair to poor condition and reflect the numerous repairs made over the years to stabilise the structure in the face of the harsh waterfront environment.

The shark-proof netted enclosure of semi-circular form is suspended on braided stainless steel cable attached to timber and concrete encased piles extending 75 metres from the beach.

Other vegetation growing around the base of the hill includes Red Bloodwoods, while Tick Bush (Kunzea ambigua) and Ball Honey Myrtle (Melaleuca nodosa) occurs on the summit.

A road winds around the hill variously enclosed by trees and exposed sandstone until it reaches the mown grass areas on the northern side, giving panoramas of the Harbour.

Such structures include the fabric of the current swimming enclosure, Sydney Water sewage pumping stations, service buildings such as the Mount Trefle workshops and visitor facilities including toilets, information shelters, BBQs, picnic shelters, signage, car parks, walking tracks, fencing, bollards, tree guards, service roads and road barriers.

[1] Its use as a private residential estate by noted colonial family of William Wentworth is demonstrated by Greycliffe House, its surviving outbuildings, landscaped setting and historic harbour view lines which are rare in consideration of their intactness.

The history of these institutions exhibit the evolution of philosophies and methods of treating infant patients and their mothers which can be demonstrated in the various alterations and additions to the buildings and landscape from this period.

Steele Point Battery and its associated outbuildings, together with the potential archaeological deposits relating to use of the site in two world wars demonstrates the strategic importance of the location to Australia's defence forces since the 1870s.

The site is of considerable historic significance as it represents the picturesque aspirations of wealthy members of society during this period, and how a European landscape aesthetic was implemented in the Australian context.

[1] Steel Point Battery, which was part of the 1870s harbour chain of defences, designed by James Barnet, occupies a prominent headland location and retains much of its layout and form.

Some buildings related to the 20th-century use of the site as a public place of leisure, such as the Dressing Pavilion, are associated with the Unemployment Relief Work Fund which employed builders during the Great Depression.

Due to its rich and diverse range of uses from pre-settlement times to the present (including uses such as a private residential estate, a colonial fort and recreational reserve), Nielsen Park may be of state significance for its ability to contribute to the understanding of the long term cultural and natural history of the harbour.

This would notably include the rubbish deposits on the western side of Mount Trefle which have been identified as highly likely to contain material of significance from the fire-damaged Greycliffe House or nearby historic properties.