Your $400/month contributions are still tax deductible in the current tax year. However, each subsequent tax year, you will be required to repay at least 1/15 of the total amount you withdrew.
In your example, let's say you used $25,000 for the home buyer's plan in the 2015 tax year. Throughout 2015, you contribute $400/month toward your RRSP.
For the 2015 tax year, you are able to claim $4800 in RRSP contributions.
For the 2016 tax year, you continue to contribute $400/month toward your RRSP. However, you are only able to claim $3133.33 in RRSP contributions, and must put at least $1666.67 toward repayment of your HBP. This will continue until you have entirely repaid the HBP. You are free to repay it early.
This CRA page has the details.