This was in apparent contravention of conditions laid down by the regulator when Macquarie bought Thames in 2006.
Macquarie told the BBC its ownership had made Thames financially stronger.
The bank and its investors sold their last share of Thames Water in March this year.
In analysis undertaken for the BBC, Martin Blaiklock, a consultant with international experience of privatised utility funding, concluded that under Macquarie's control, the total returns made by the bank and its investors from Thames Water averaged between 15.5% and 19% a year.
Mr Blaiklock has 40 years of experience in such matters and said these returns were "twice what one would normally expect".
Macquarie disputed Mr Blaiklock's estimate of the total returns paid by Thames to Macquarie and its investors.
The bank referred the BBC to the 12.3% internal rate of return earned by its European Infrastructure Fund, of which their investment in Thames Water was a part.
Mr Blaiklock says that's not an appropriate measure of what Thames paid.
These disclosures follow scrutiny of the possible financial causes of Thames water's extensive pollution of the Thames, and other rivers, with untreated sewage between 2012 and 2014.
The company was under Macquarie's control at the time.
The Environment Agency prosecuted Thames Water over the pollution and, in March this year, the company was fined £20m - a record for such an offence.
At Aylesbury Crown Court, Judge Francis Sheridan said there had been "inadequate investment, diabolical maintenance and poor management".
Assessing incidents as being in the "reckless" or "borderline deliberate" category, Judge Sheridan concluded: "Knowledge of what was going on went very high indeed."
Thames Water accepts that there was a failure of management, but denies that there was a link between the sewage pollution and high investor returns. The company points out that there have been "significant management changes" since that time.
The transactions which culminated in Thames Water having the additional £2bn of debt on its books took place inside a network of companies set up by Macquarie at the time it bought Thames Water.
A consultation paper published in February 2007 by the water regulator Ofwat showed that Macquarie and its investors paid £5.1bn for Thames Water, of which £2.8bn was money Macquarie had borrowed to help fund the purchase.