"It's probably one of the poorest sites in our whole portfolio and a top priority for replacement. While they don't complain about it, we would love to see them in better accommodation than they're in at the moment."
He said police discussion with iwi had been going well and it aimed to get started on the project by the end of the year.
The Ministry of Justice revealed in October last year the $50 million project for a new courthouse, along with a new police station, to go where the former UCOL campus was located, in between Bell, Dublin, Wicksteed and Liverpool Sts.
There had been a recent increase in family violence in Whanganui and Coster said that aligned with a lot of the rest of the country.
"We see family violence as a really key opportunity ... because we know a large proportion
of youth offenders would have left family violence in the home as well as gang members."
Police had a responsibility to deal with some aspects of family violence but there was also a need to partner with other organisations.
"There are a complex range of factors that play into this and often the needs to be addressed do sit around things like housing, things like parenting skills."
Coster and Police Minister Chris Hipkins are on a national tour of the country - starting in Levin on Monday before visiting Feilding and Whanganui. On Tuesday they were in Ohakune and National Park.
The pair wanted to hear from officers about how they could be best supported to do their jobs.
Hipkins said there were a range of similar issues raised across the country.
He said in areas like Ohakune, where staff were more isolated, there was a "unique set of challenges".
"We've got a range of issues being raised across the country that are quite similar and then in different areas you get a slightly different take on things.
"Gang crime and gang activity is a theme right across the country."
Regular issues around domestic and family violence were common and there were issues with boy racers cropping up in some areas, the minister said.
Coster said he believed police were doing well to deal with theft.
"We are really mindful of the impact of offending on retail businesses.
"One of the initiatives we started recently is our national retail investigations unit which is really aimed at trying to identify recidivist offenders and try to pull together packages of offending for investigation and prosecution."
That would help deal with the more prolific offenders, Coster said.
"Certainly very sympathetic to those small businesses, family-owned businesses and the like that might be impacted by that sort of offending.
"Staff do a good job, I think, in terms of trying to support those places and trying to follow up as best they can."