The Australian National Dictionary Centre (Australian National University, Canberra, Australian Capital Territory) writes that the Australian-English phrase bottom of the harbour denotes a tax avoidance scheme consisting in buying a company with a large tax liability, converting the assets to cash, and then ‘hiding’ the company by, for example, selling it to a fictitious buyer: thus the company (and often its records) vanished completely—figuratively sent to the bottom of the harbour (originally Sydney Harbour)—with an unpaid tax bill.

Chiefly used attributively, this phrase occurs, for example, in Diamond Dove (Melbourne: Text Publishing, 2006), a novel by Australian author Adrian Hyland (born 1954):

I’d been called up for jury duty. The feller in the dock was some fabulous creature—part lawyer, part farmer—who’d been caught in a bottom-of-the-harbour tax avoidance scheme. As I watched him being sworn in, a thin smirk on his fat face, I was struck by a feeling of déjà vu. We went through the motions, but he got off of course. They always do.

The earliest occurrences of the Australian-English phrase bottom of the harbour that I have found are as follows, in chronological order:

1-: From The Sydney Morning Herald (Sydney, New South Wales, Australia) of Thursday 27^th November 1980:

Howard sets sights on $58m tax evader
New law on ‘bottom of harbour’ schemes
From Greg Bright

Canberra—A Sydney businessman who has masterminded the evasion of almost $60 million in tax over the past 15 months is still operating evasion schemes.
This was revealed yesterday by the Treasurer, Mr Howard, as he introduced a new tax evasion law in Parliament aimed at stamping out tax scheme promoters.
[…]
The Sydney businessman, who was not named, […] is the biggest known evasion promoter operating in Australia.
He and similar “straw” company users buy cashed-up businesses from which all the assets are transferred, leaving only tax liabilities.
The directorships and shareholders are changed (usually to known criminals or vagrants) and the company records afterwards destroyed. It is known in tax circles as “the bottom-of-the-harbour” scheme.

2-: From The Sydney Morning Herald (Sydney, New South Wales, Australia) of Wednesday 3^rd December 1980:

Lawyers try to delay tax law
From Greg Bright

Canberra—A group of lawyers has launched an intensive lobbying effort to delay the passage of a new tax-evasion law which provides jail sentences for offences.
[…]
The bill provides for jail sentences of up to five years and fines of up to $50,000 for tax evasion stripping companies or trusts of their assets.
Records are often destroyed in what have been called “bottom of the harbour” schemes. One Sydney businessman is known to have masterminded the evasion of $58 million in tax over 15 months.

3-: From The Sydney Morning Herald (Sydney, New South Wales, Australia) of Wednesday 17^th February 1982:

Sydney company principal at dockers’ inquiry
$1m tax-avoidance scheme
By Greg Wilesmith

A Sydney company that marketed tax-avoidance schemes grossed up to $1.5 million in 1979-80 selling just one scheme, the Royal Commission into the Federated Ship Painters and Dockers’ Union was told yesterday.
Mr Frank Ward, one of the principals of Ward, Knight and Dunn, said between $1 million and $1.5 million was earned by charging companies that used the scheme 10 per cent of the tax they would have paid.
[…]
[…] Mr Douglas Meagher, QC, […] told the commission that the registered office of Rolls Corporation at South Yarra was the office of about 40 companies registered by the Victorian Corporate Affairs Commission.
Mr Meagher said the office appeared to have been abandoned and considerable mail had piled up there, including letters from the Taxation Office.
It might be, he said, what is known in the tax business as a “bottom of the harbour scheme.”
This has been referred to previously as a situation where companies owing tax change hands and the Taxation Office is unable to find the directors.
Mr Ward denied any involvement in “bottom of the harbour” schemes.