News of the decision, made at the end of last year, follows the third sewage spill into Lake Burley Griffin in seven months.

Mitchell-based plumbing business Dr Drains, owned by Jeremy Kingston, had been employed by ACTEW Water for about eight years on a rolling contract basis to use a herbicide to destroy roots in Canberra's sewers and prevent them from growing back.

Mr Kingston said the contracts had been worth between $200,000 and $500,000, depending on the amount of work done.

He had been planning to again tender for a contract towards the end of last year and was told there was none available.

A spill from one of ACTEW Water's pipes forced the closure of Yarralumla Bay in Lake Burley Griffin on Monday last week, and in November the same spot was closed to swimmers after sewage was spotted flowing into the lake.

In September the bay was closed after a sewage leak, but last week an ACTEW Water spokeswoman would not comment on whether the leaks had originated from the same site, saying the matter was being investigated.

ACTEW is also facing government scrutiny after it was revealed managing director Mark Sullivan's $855,000 pay deal was reported as $621,000 in September 2011 in an annual report and an ACTEW letter to the shareholders.

And the Independent Competition and Regulatory Commission released a draft report on ACTEW's water and sewerage services in February, recommending the price of water fall by 16.9 per cent and sewerage charges fall by 24 per cent, prompting Mr Sullivan to warn he may need to cut staff to make up for the loss of revenue.

The National Water Commission's National Performance Report 2011-2012 compared ACTEW's number of sewer main breaks and chokes with nine other urban water utilities with more than 100,000 connected properties. Six of the nine utilities had fewer breaks and chokes than ACTEW, although its perfomance had improved considerably since 2009-10, when it had more than any other similarly sized utility.

Mr Kingston said ACTEW Water had always been pleased with his work and he suspected the commission's involvement in ACTEW's finances was behind the decision not to offer another contract.

''The suburbs we probably should be treating are the ones we know have roots in them and haven't become a major problem yet and caused damage to the infrastructure,'' he said.

An ACTEW Water spokeswoman said the company had a significant capital maintenance and investment program, which involved the systematic cleaning and inspection of suspect or high-risk sewer pipes to keep them free of roots.

''ACTEW Water did employ Dr Drains on a rolling contract basis. However, at present we have decided to return to more tried and tested means,'' she said.

The spokeswoman said the company had spent $21 million in replacing ageing and broken parts of Canberra's sewer network during its current investment program, including repairing at-risk pipes before they became a problem.

''ACTEW Water is always looking, where possible, to improve the service we provide to our customers,'' she said.

Mr Kingston said he used a contact herbicide, which killed invasive roots but not trees, and retreatment was required every three years.