Lawyers representing foster children are being kept in the dark on decisions made about their client, according to Legal Aid.
Ms Hiles gave evidence on Wednesday to the ACT Legislative Assembly inquiry into information sharing in the territory's care and protection system.
The probe was sparked by the government's wrongful removal of five Aboriginal children from their mother in 2013, a case described by some as being reminiscent of the Stolen Generation.
The inquiry on Tuesday heard that carers were struggling to access critical medical information about their foster children due to legal and bureaucratic barriers.
Ms Hiles told Wednesday's hearing that despite being a child's legal representative and advocate, lawyers were often not consulted on, or even made aware of, decisions made about their client.
For example, lawyers might only learn that their client had been moved to another home when court documents were filed, or through chance conversations with government solicitors, she said.
Layers of bureaucracy also slowed the process of, or prevented entirely, a lawyer getting permission to speak with a child.
Ms Hiles said a member of her team once visited a client at school "because there was no other option".
Committee member Vicki Dunne questioned how a lawyer could effectively advocate for a child if they were "deprived" of information vital to the case.
Ms Hiles responded: "You can't".
"What it [denying access to information] is elongates the court process," she said.
"If we aren't able to access the information that we need to in order to give proper legal advice ... we are in a position where the process is frustrated to the point where nobody can effectively do their job."
Ms Hiles was particularly concerned that so many types of decisions made by child protection authorities, including where a foster child was placed, were not subject to external review under territory laws.
The hearing also heard evidence from Women's Legal Centre ACT principal solicitor Claudia MacLean, whose organisation provides support to women in child protection cases,
In its written submission to the inquiry, the centre argued secrecy provisions in ACT child protection laws were "relied upon" by authorities to "limit the ability of a women to understand and respond to child protection concerns".
On Wednesday, Ms MacLean said senior officials in the child protection agency had acknowledged the need to change its attitude on sharing information with women.
But Ms MacLean said lower-level officers remained very "risk averse", and were therefore more likely to withhold information.
The constant churn of staff made it more difficult to "change the culture" inside the agency, she said.