The September 2011 fire at a Mitchell oil recycling plant sent fireballs and a plume of smoke some 200 metres into the air, prompting an independent review completed in April 2012 that was not released until 2013.

Six years on, the Legislative Assembly on Tuesday passed laws to finally force Canberra's warehouse owners to apply for development approvals to store large amounts of chemicals in their buildings.

But Mr Gentleman could not explain the delay on Tuesday, instead saying he was "seeking an explanation from the directorate as to why it has taken this amount of time".

The changes mean warehouse owners storing large amounts of dangerous substances now have six months to make a development application to allow such chemicals to be stored on site.

Environment, planning and sustainable development directorate executive director of planning delivery Brett Phillips said the changes will not change the existing protections under the ACT's dangerous substances laws.

The specific amount of chemicals that will warrant a development application will vary depending on the substance.

But Mr Phillips said it would only apply to large warehouses and would not affect small businesses like swimming pool maintenance firms storing smaller amounts of such chemicals.

"We'll be going out to businesses to provide information in relation to their sites over the next few months and we'll also provide them with contact details in relation to how they can be certain they'll comply," he said.

The reforms also remove a requirement for developers to complete a full environmental impact statement on contaminated land, to cut regulations where the development does not actually impact the contaminated site.

United Firefighters Union secretary Greg McConville said while the changes to dangerous substances' storage were welcome, he was concerned the contaminated sites change would lower the bars to conducting EIS' on development sites.

"The basis on which the government is saying you shouldn't have to do an EIS in every case is that it's, quote 'an arbitrary requirement which does not permit a level of assessment commensurate with the level of risk'.

"But how do you know what that level of risk is if you haven't done a full assessment?"

While those changes will instead require government officers to provide an 'environmental significance opinion', that would be based on a developers' assessment of the site.

Mr Phillips said that referrals for an environment impact assessment of contaminated sites was mandatory, and that would "still be a level of scrutiny from our side".

A third prong of the changes will see all Territory Plan variations - except technical amendments - referred to an Assembly committee to spend 20 days to consider if a public inquiry was needed to examine the variation.

The committee could then decide to hold an inquiry, which would delay a decision on the variation for at least six months, or not to hold an inquiry if the change minor.

While the Canberra Liberals and ACT Greens backed the government's changes, with amendments; Greens MLA Caroline Le Couteur said she still wanted more consultation around 'technical amendments' of the Territory Plan.

Ms Le Couteur currently has a private members bill before the Assembly that would only allow technical amendments to be made if the planning authority deemed them uncontroversial, a bill yet to be debated.